Deterrence isn't what it used to be. In the second half of the twentieth century, it was the backbone of U.S. national security. Its purpose, logic, and effectiveness were well understood. It was the essential military strategy behind containing the Soviet Union and a crucial ingredient in winning the Cold War without fighting World War III. But in recent decades, deterrence has gone astray, and U.S. defense policy is worse for the change.

Since the Cold War ended, the United States has clung to deterrence where it should not have, needlessly aggravating relations with Russia. More important, it has rejected deterrence where it should have embraced it, leading to one unnecessary and disastrous war with Iraq and the risk of another with Iran. And most important, with regard to China, Washington is torn about whether or not to rely on deterrence at all, even though such confusion could lead to a crisis and a dangerous miscalculation in Beijing.

Mistakes in applying deterrence have come from misunderstandings about the concept itself, faulty threat assessments, forgetfulness about history, and shortsighted policymaking. Bringing these problems into focus can restore faith in deterrence where it has been lost, lower costs where the strategy has been misapplied, and reduce the danger of surprise in situations where the risk of conflict is unclear.

Deterrence is a strategy for combining two competing goals: countering an enemy and avoiding war. Academics have explored countless variations on that theme, but the basic concept is quite simple: an enemy will not strike if it knows the defender can defeat the attack or can inflict unacceptable damage in retaliation.

At best, applying deterrence when it is unneeded wastes resources. At worst, it may provoke conflict rather than hold it in check. And even when deterrence is appropriate, it might not work -- for example, against an enemy who is suicidal or invulnerable to a counterattack. Thus, it is more useful against governments, which have a return address and want to survive, than against terrorists who cannot be found or who do not fear death. Deterrence is also a weak tool in the increasingly important realm of cyberspace, where it can be extremely difficult to be absolutely sure of an attacker's identity.

When the United States does choose to apply deterrence and is willing to fight, the deterrent warning must be loud and clear, so the target cannot misread it. Deterrence should be ambiguous only if it is a bluff. One of the biggest dangers, however, comes in the reverse situation, when Washington fails to declare deterrence in advance but then decides to fight when an unexpected attack comes. That kind of confusion caused the United States to suddenly enter both the Korean War and the Gulf War, despite official statements in both cases that had led the aggressors to believe it would not.

Deterrence is not a strategy for all seasons. It does not guarantee success. There are risks in relying on it and also in rejecting it when the alternatives are worse.

UNNECESSARY ROUGHNESS

To Moscow, it must seem that the Cold War is only half over, since the West's deterrence posture, although muted, lives on. During the Cold War, deterrence was vital because the Soviet threat seemed huge. Moscow's military capabilities included some 175 divisions aimed at Western Europe and close to 40,000 nuclear weapons. Soviet intentions were much debated, but they were officially assumed to be very hostile. The West's response was to deploy ample military counterpower via NATO and the U.S. Strategic Air Command. And for more than 40 years, deterrence held. Despite tense crises over Berlin and Cuba and proxy conflicts in the Third World, Moscow never dared unleash its forces directly against the West. Doves doubted that so much deterrence was necessary, but hawks were reassured that against a potent threat, deterrence did not fail.

Yet implicit deterrence persisted after the West's victory because of demands from members of the old Warsaw Pact that joined NATO, the retrograde politics of the post-Soviet Russian state, and sheer force of habit. The 2012 Republican U.S. presidential candidate Mitt Romney was only channeling a common view when he said that Russia remained the United States' "number one geopolitical foe."

Although most of the remaining U.S. military infrastructure committed to NATO provides logistical support for missions "out of area," and despite the tightening of the U.S. defense budget, two U.S. brigade combat teams are still stationed in Europe. These might seem only symbolic, but together with NATO's expansion, they appear aimed at Moscow. The United States and Russia also continue to negotiate with each other over their nuclear arsenals. But there is no reason for formal arms control between countries unless they fear each other's forces, feel the need to limit what they could do to each other in the event of war, and want to institutionalize mutual deterrence.

These continuities with the Cold War would make sense only between intense adversaries. Washington and Moscow remain in an adversarial relationship, but not an intense one. If the Cold War is really over, and the West really won, then continuing implicit deterrence does less to protect against a negligible threat from Russia than to feed suspicions that aggravate political friction. In contrast to during the Cold War, it is now hard to make the case that Russia is more a threat to NATO than the reverse. First, the East-West balance of military capabilities, which at the height of the Cold War was favorable to the Warsaw Pact or at best even, has not only shifted to NATO's advantage; it has become utterly lopsided. Russia is now a lonely fraction of what the old Warsaw Pact was. It not only lost its old eastern European allies; those allies are now arrayed on the other side, as members of NATO. By every significant measure of power -- military spending, men under arms, population, economic strength, control of territory -- NATO enjoys massive advantages over Russia. The only capability that keeps Russia militarily potent is its nuclear arsenal. There is no plausible way, however, that Moscow's nuclear weapons could be used for aggression, except as a backstop for a conventional offensive -- for which NATO's capabilities are now far greater.

Russia's intentions constitute no more of a threat than its capabilities. Although Moscow's ruling elites push distasteful policies, there is no plausible way they could think a military attack on the West would serve their interests. During the twentieth century, there were intense territorial conflicts between the two sides and a titanic struggle between them over whose ideology would dominate the world. Vladimir Putin's Russia is authoritarian, but unlike the Soviet Union, it is not the vanguard of a globe-spanning revolutionary ideal.

The imbalance of capabilities between NATO and Russia does not mean that Moscow's interests are of no concern, or that the United States can rub the Russians' noses in their military inferiority with impunity. Russia is still a major power whose future policies and alignment matter. Indeed, if Russia were to align with a rising China, the strategic implications for the United States would not be trivial. Too many Americans blithely assume that continued Chinese-Russian antagonism is inevitable; in fact, Japan, NATO, and the United States are providing China and Russia with incentives to put aside their differences and make common cause against pressure from the West.

Even absent a Chinese-Russian partnership, confronting Russia poses unnecessary risks. The only unresolved territorial issues in the region are more important to Moscow than to the West, as the 2008 miniwar between Georgia and Russia demonstrated. If NATO were to expand deterrence even further by admitting Georgia as a member -- a move the Obama administration supports in principle, as did the George W. Bush administration -- it would be a direct challenge to Moscow's protection of secessionist regions in the country. It would constitute a frank statement that Russia can have no sphere of interest at all, one of the usual prerogatives of a major power. NATO would thus finish the job of turning deterrence into forthright domination -- precisely what China and the Soviet Union used to falsely claim was the real intention of the West's deterrent posture. In the worst case, admitting Georgia into NATO could be the last straw for Russia, precipitating a crisis.

The cost of either of those outcomes would be higher than the price of a more decisive Western military stand-down and an end to talk of expanding NATO further. Stable peace with an uncongenial regime in Moscow should be a higher priority than unconditional backing for Russia's closest neighbors. Ultimately, however, as long as NATO is an alliance that excludes Russia, rather than a genuine collective security organization, which would have to include Russia, Moscow will inevitably interpret its very existence as a threat. The consolidation of peace in Europe will not be complete as long as practically every European country belongs to NATO except Russia. The idea of Russian membership is fanciful so far; there is no movement for it in the West, nor any indication that Moscow would join even if invited. But Russian claims that NATO is a threat would be easier to discredit if its members appeared willing to consider inviting Russia to join, if it gets back on the path to democracy.

LESSONS UNLEARNED

Too much deterrence of Russia is a mistake, but not as serious as the opposite mistake: rejecting deterrence where it is badly needed. That mistake is harming U.S. efforts to cope with nuclear proliferation and, most particularly, Iran. Instead of planning to deter would-be proliferators, U.S. policymakers have developed a preference for preventive war. They now seem to fear that deterrence is too weak to deal with radical regimes, forgetting that the precise purpose of deterrence is to counter dangerous enemies, not cautious ones. This preference is especially troubling because it continues even after two painful experiences with Iraq that vividly highlighted why deterrence is the better choice.

Deterrence played no role in the run-up to the first major conflict after the Cold War, the 1990-91 Gulf War. The assault by Saddam Hussein on Kuwait was widely misread as showing that he was undeterrable. It showed no such thing, however, because the United States had not tried to deter him. Had Saddam known that invading Kuwait would spur Washington to launch a decisive war against him, he surely would have refrained. But the administration of George H. W. Bush never made such a threat, and the dictator was left free to miscalculate.

Bush had been unprepared to make a deterrent threat because an Iraqi invasion of Kuwait was a totally unexpected contingency. This situation was exactly the same as the one that had produced an unexpected and avoidable war 40 years earlier. In 1949, U.S. Army General Douglas MacArthur publicly stated that South Korea did not fall within the U.S. defense perimeter in Asia; the following year, U.S. Secretary of State Dean Acheson made comments to the same effect. These statements reflected the fact that the United States was planning for a third world war, in which Korea would be a low priority -- which is why President Harry Truman was completely surprised when the North attacked the South in the absence of a broader war.

In 2003, George W. Bush did not have the excuse of surprise. He deliberately chose not to rely on deterring Iraq, deciding instead to start a war immediately in order to prevent Iraq from possibly using weapons of mass destruction sometime in the future. The result was a disaster.

It is impossible to know whether the alternative of relying on deterrence to keep Saddam in check would have produced a bigger disaster, as the war's instigators asserted. There is no evidence, however, that Saddam could not have been deterred indefinitely. He had indulged in wanton aggression against Iran in 1980 and Kuwait a decade later, but these were cases in which he had reason to believe that he faced no daunting counterpower. He was a reckless bully, but not suicidal. He never attacked in the face of a U.S. threat to respond, nor did he use his chemical or biological weapons even in defense against the U.S. assault in 1991, when Washington exercised deterrence by warning of devastating retaliation in the face of such an attack.

American fears of Saddam -- and now of Iran's leaders -- seem exaggerated in light of the United States' experience during the Cold War. Presidents considered but rejected preventive war against Mao and Stalin, who seemed even more fanatical and aggressive in their time than today's foes. Mao issued chilling statements unmatched by anything yet heard from leaders in Tehran -- for example, that the prospect of nuclear war "is not a bad thing" because defeating capitalism would be worth losing up to two-thirds of the world's population.

Considering the positive results of Cold War containment and the awful miscarriage of a preventive strategy against Iraq, one might have expected U.S. policymakers to find deterrence attractive as a fallback strategy to deal with Iran if the Islamic Republic could not be dissuaded from developing nuclear weapons. After all, that is precisely how Washington has dealt with a nuclear-armed North Korea. But U.S. and Israeli leaders have convinced themselves that Iran might one day use nuclear weapons for aggression -- irrationally and without provocation. There is no evidence, however, that the Iranian leadership has any interest in national suicide, the likely consequence of an Iranian nuclear first strike. Iran has supported terrorism, justifying it as a response to American and Israeli covert warfare. But however aggressive its motives, the revolutionary regime in Tehran has never launched a regular war against its enemies.

Nevertheless, rather than planning to deter a prospective Iranian nuclear arsenal, the United States and Israel have preferred preventive war. Although many still hope to turn Iran away from nuclear weapons through sanctions and diplomacy, the debate within and between the United States and Israel over what to do if Iran moves to produce a bomb is about not whether to attack but when. U.S. President Barack Obama has firmly declared that he has not a "policy of containment" but rather "a policy to prevent Iran from obtaining a nuclear weapon," and other administration officials have repeatedly emphasized this point. As promises in foreign policy go, this one is chiseled in stone. Backing down from it when the time comes would be the right thing to do but would represent an embarrassing retreat.

The logic behind rejecting deterrence is that Tehran might decide to use nuclear weapons despite facing devastating retaliation. The risk can never be reduced to zero, but there is no reason to believe that Iran poses more danger than other nasty regimes that have already developed nuclear weapons. The most telling example is North Korea. Although the American public has not paid nearly as much attention to North Korea, Pyongyang's record of fanatical belligerence and terrorist behavior over the years has been far worse than Tehran's.

Refusing to accept an iota of risk from Iran ignores the massive risks of the alternative of initiating war. Leaving aside the danger of being blind-sided by unanticipated forms of Iranian reprisal -- for example, the use of biological weapons -- the obvious risks include Iranian retaliation by overt or covert military means against U.S. assets. The results of the initially successful assault on Iraq in 2003 are a reminder that wars the United States starts do not necessarily end when and how it wants them to. Indeed, the records of the United States and Israel suggest that both countries tend to underestimate the prospective costs of the wars they enter. Washington paid fewer costs than expected during the Gulf War but faced a far higher bill than anticipated in Korea, Vietnam, Kosovo, Afghanistan, and the second war against Iraq. Israel suffered less in the 1967 Six-Day War than expected but was badly surprised by the costs of the 1973 Yom Kippur War, the 1982 Lebanon war, and the 2006 war against Hezbollah.

Launching a war against Iran would also have negative spillover effects. First and foremost, short of an accompanying ground invasion and occupation, an air attack could not guarantee an end to Iran's pursuit of nuclear weapons; it could guarantee only a delay and would almost certainly drive the Iranians to commit more fervently to building a bomb. If Iran's capabilities were only temporarily degraded but its intentions were inflamed, the threat might become worse. Striking first would also fracture the international coalition that now stands behind sanctions against Iran, undercut opposition to the regime inside the country, and be seen throughout the world as another case of arrogant American aggression against Muslims.

Those costs might seem justifiable if launching a war against Iran dissuaded other countries from attempting to get their own nuclear deterrents. But it might just as well energize such efforts. George W. Bush's war to prevent Iraq from getting nuclear weapons did not dissuade North Korea, which went on to test its own weapons a few years later, nor did it turn Iran away. It may have induced Libyan leader Muammar al-Qaddafi to surrender his nuclear program, but a few years later, his reward from Washington turned out to be overthrow and death -- hardly an encouraging lesson for U.S. adversaries about the wisdom of renouncing nuclear weapons.

One reason U.S. leaders might be reluctant to apply deterrence these days is that the strategy's most potent form -- the threat to annihilate an enemy's economy and population in retaliation -- is no longer deemed legitimate. In 1945, hardly any Americans objected to the incineration of hundreds of thousands of Japanese civilians, and throughout the Cold War, few objected to the principle of killing on an even greater scale in retaliation for a Soviet attack. But times have changed: today, post-Cold War norms and Pentagon lawyers have put the idea of deliberately targeting civilians thoroughly out of bounds. It would be difficult for the U.S. government to declare that if one Iranian nuclear weapon was detonated somewhere, it would kill millions of Iranians in return.

But that inhibition should hardly be a reason to prefer starting a war, nor does it cripple deterrence. An acceptable variant would be to threaten not to annihilate Iran's population but to annihilate its regime -- the leaders, security agencies, and assets of the Iranian government -- if it used nuclear weapons. Although in practice, even a discriminating counterattack of that kind would result in plenty of collateral damage, U.S. planners could credibly make the threat and could reinforce it by pledging to invade Iran as well -- a step that would be far more reasonable to take after an Iranian nuclear strike than it was against Iraq in 2003. And even if legal concerns constrained the United States from massively retaliating against Iranian civilians, Israeli leaders would surely be willing to do so if Iran attacked Israel with nuclear weapons, since Israel's national existence would be at stake. Those mutually reinforcing threats -- that the fruits of the Iranian Revolution and even Iranian society itself would cease to exist -- would be an overwhelming restraint on Tehran.

A nuclear-armed Iran is an alarming prospect. But there is no sure solution to some dangers, and this challenge presents a strategic choice between different risks. There is simply no real evidence that war with Iran would yield any more safety than handling the problem with good old deterrence.

MIXED SIGNALS

The most dangerous long-term risk posed by Washington's confusion over deterrence lies in the avoidance of a choice one way or the other about the strategy when it comes to China. Washington needs to determine whether to treat Beijing as a threat to be contained or a power to be accommodated. U.S. policymakers have long tried to have it both ways. Such incoherence is politically natural but harmless only so long as no catalyst exposes the contradiction. It will thus prove unsustainable unless China decides to act indefinitely with more humility than any other rising power in history has and unless it feels less sense of entitlement than the United States does itself.

One influential view has held that deterrence is a nonissue for U.S.-Chinese relations because the two states' economic interdependence precludes military conflict. In this view, confrontation is nonsensical, so preparing for it only risks turning it into a self-fulfilling prophecy. The opposing view, that China's rising power is a threat that must be countered militarily, has been gaining but has not been turned into explicit policy. Meanwhile, the Obama administration's declared "pivot," or "rebalancing," of American military power toward Asia has not been accompanied by consistent signals about where, when, why, or how U.S. armed forces would be sent into combat against China, nor is there a clear operational rationale for basing a contingent of U.S. marines in Australia, the most concrete symbol of the pivot. The problem is not that deterrence has been inappropriately rejected or embraced but that it is muddled.

Adding to this lack of clarity, Washington continues to ignore the question of when and how Beijing's long patience about resolving Taiwan's status could end. China has always made clear that the question of reunification is a matter of when, not whether. For years, Washington has kicked the can down the road by warning Taipei not to declare independence, a provocation that Beijing has said would trigger military action. But when asked in 2001 what the United States would do to defend Taiwan, Bush declared, "Whatever it takes." In effect, U.S. policy has evolved into a promise to defend Taiwan as long as it is a rebellious province of China, but not if it is a separate country. This stance strikes some experts as a clever solution -- but it defies most Americans' common sense, sends an ambiguous signal to Beijing, and thus undermines Washington's readiness for a crisis.

Meanwhile, conflicts brew, such as the recent revival of tensions over disputed islands in the South China Sea. Preoccupied with other strategic challenges, Washington is drifting toward unanticipated confrontation, without a clear decision about the circumstances in which it would be willing to go to war with China. This distraction and hesitancy prevent the sending of clear warnings to Beijing of U.S. "redlines," increasing the risks of an inadvertent crisis, miscalculation, and escalation.

Chinese and Philippine naval maneuvers near disputed islands in mid-2012 were a wake-up call, and subsequent jockeying by China and Japan over the even more dangerous disagreement over who owns the Diaoyu/Senkaku Islands put Washington's confusion into stark relief. The initial U.S. response to the latter dispute was a disturbing contradiction: "We don't take a position on the islands, but we do assert that they are covered under the treaty," a State Department spokesperson declared, referring to the mutual security agreement between Japan and the United States. Secretary of Defense Leon Panetta then said that the United States would not take sides in regional disputes over territory and also claimed that although U.S. strategic rebalancing toward Asia is more than mere rhetoric, it is not a threat to China.

All of this is ambivalent deterrence: rhetorical bobbing and weaving rather than strategic planning. It is a dangerous practice, projecting provocation and weakness at the same time. Washington signals Beijing not to occupy the various islands but does not threaten to block it from doing so, even while assuring Tokyo that the United States is treaty-bound to defend the islands. Subsequent clarifications or secret statements to either capital might mitigate the contradiction, but the public posture subverts U.S. credibility. It invites Chinese leaders to see the United States as a paper tiger that may fold in an escalating crisis. Yet in such a crisis, under the pressure of events for which it has not prepared, Washington might surprise its opponent by choosing war, for the same reasons it did so after the invasion of South Korea in 1950 and Kuwait in 1990.

There are two logical long-term alternatives to this risky confusion. One is to make a clear commitment to contain China, meaning that Washington would block Beijing from expanding its territory through either military action or political coercion. This sounds precipitate, because China sees containment as an aggressive threat. So Washington would have to express this commitment carefully, emphasizing the defensive aim of securing the status quo, not challenging China's rights. The benefit of this alternative would be to make deterrence harder to mistake and thus more effective, allowing for brighter redlines, which would reduce the odds of an unanticipated game of chicken producing a war neither side wants. But the costs would be very high: a new Cold War and the disruption of advantageous cooperation on many issues. The United States would also have to decide once and for all whether it is willing to go to war with China over Taiwan. At present, there is no serious discussion about this, let alone consensus, among either U.S. voters or the foreign policy elite in Washington.

If a "red light" strategy of containment is unnecessary or too costly, the second, opposite alternative is accommodation -- in effect, a green light. Accommodation would make sense if Beijing's ambitions were limited and likely to stay limited, if its growing power was not in danger of being derailed, and if the United States preferred disregarding the interests of its allies to growing conflict with an emerging superpower -- all big ifs. In accommodating Beijing, Washington would recognize that as China becomes a superpower, it will naturally feel entitled to the prerogatives of a superpower -- most obviously, disproportionate influence in its home region. And Washington would have to accept that disputes over minor issues will be settled on China's terms rather than those of its weak neighbors. The big obstacle to this alternative would be the conflict over Taiwan, a far more consequential dispute than those over the uninhabited rocks whose status provoked tensions last year. And just as there is no consensus for containment, Americans loathe anything that reeks of appeasement.

Given the unattractiveness of either alternative, it is no surprise that Washington has finessed the question. Incoherent compromise is a common and sometimes sensible diplomatic strategy. In Asia, however, it means underestimating the risks of drift and indecision if Chinese power grows and Chinese restraint diminishes. U.S. policy now amounts to a yellow light, a warning to slow down, short of a firm requirement to stop. Yellow lights, however, tempt some drivers to speed up.

There is no pain-free solution to the problem posed by China's ascent, unless Taiwan surrenders peacefully. Kicking the can down the road might work for a long time, but only as long as Chinese forbearance lasts. If a crisis erupts, ambivalent deterrence may cause conflict rather than prevent it. It might prove too weak to make Beijing swerve first, but strong enough to keep Washington from swerving also, thus causing a collision. The only solution is a clear strategic decision about whether the United States will accept China's full claims as a superpower when it becomes one or draw clear redlines before a crisis comes.

Deterrence is not disastrous when applied mildly, although unnecessarily, to Russia, but it does have negative effects. Deterrence is not a sure thing against Iran, but it beats starting a war, especially a war that could make the ultimate threat worse. And in the face of the serious long-term policy dilemma posed by China, opting for or against deterrence is an extraordinarily hard choice, but avoiding the choice makes the dilemma ever more dangerous. Reducing future risks requires paying some immediate costs.

Getting deterrence back into focus will help fix these strategic problems. In the Cold War, the strategy was so ingrained and pervasive an element of U.S. strategy that "deterrence" became a buzzword used to justify everything in defense policy. In recent years, however, it has almost vanished from the vocabulary of strategic debate. U.S. policymakers need to relearn the basics of deterrence and rediscover its promise as a strategy in the right circumstances, while recognizing its drawbacks in others. The alternative of continued confusion will not matter, until it does -- for example, whenever Beijing decides that the day has come for the changes it has always said were only a matter of time.