The Syrian regime’s horrific chemical weapons attack Tuesday has totally inverted the U.S. policy debate on Syria. The administration of U.S. President Donald Trump has whipsawed—in less than a week—from acquiescing to the continued rule of Syrian President Bashar al-Assad to threatening some unspecified unilateral intervention in Syria.

The rhetoric of top U.S. officials suggests the United States may be headed toward a major new escalation in Syria. If so, any threatened U.S. action should have the narrow, clearly articulated objective of halting the use of chemical weapons by the Assad regime and deterring their use globally. It should not be aimed at achieving a negotiated political resolution to the Syrian war or any gradation of regime change—goals that were rightly judged impractical as recently as Monday.

If the United States is going to act in Syria, specificity, clarity, and realism are its friends. Vague maximalism is not.

If the United States is going to act in Syria, specificity, clarity, and realism are its friends. Vague maximalism is not.

CROSSING THE LINE

On Tuesday, a government air strike on the town of Khan Shaykhun, in Syria’s rebel-held northern Idlib Province, released toxic gas that reportedly left at least 70 dead and hundreds more affected. The incident looks set to be the deadliest chemical weapons attack in Syria since the Assad regime’s August 2013 nerve gas attack on the Damascus suburbs, which killed nearly 1,500 people.

The Syrian military has denied using chemical weapons, instead blaming “terrorist groups,” and Russia has claimed that a Syrian regime air strike in fact struck an insurgent chemical weapons depot. Yet the United States has asserted definitively that the regime was responsible for the attack. The Assad regime has recently used toxic gas against other insurgent-held towns, UN inspectors have found, albeit chlorine gas rather than the more deadly sarin that is believed to have been used on Tuesday.

In response to the attack, the U.S. position on Assad flipped overnight. “My attitude toward Syria and Assad has changed very much,” Trump said on Wednesday. Asked by one reporter if the Khan Shaykhun attack crossed a “red line,” Trump replied it “crosses many, many lines, beyond a red line.”

Just one week before, U.S. Ambassador to the United Nations Nikki Haley told reporters, “Our priority is no longer to sit there and focus on getting Assad out.” But by Wednesday, Haley had taken a much harder line, demanding that Russia rein in its Syrian client or risk unilateral U.S. action. “When the United Nations consistently fails in its duty to act collectively,” she said, “there are times in the life of states that we are compelled to take our own action.”

U.S. officials may now have, in effect, committed the United States to action in Syria.

Their statements recall then-President Barack Obama’s 2012 warning that the use of chemical weapons by the Assad regime would cross a redline. After the Assad regime crossed that line in August 2013, the Obama administration backed off an initial threat of military action and instead concluded an agreement with Russia to peacefully eliminate (seemingly) all of Syria’s chemical arsenal.

In his remarks Wednesday, Trump specifically criticized Obama’s handling of the 2013 episode. “When [Obama] didn’t cross that line after making the threat,” he said, “I think that set us back a long ways, not only in Syria, but in many other parts of the world, because it was a blank threat.”

THE OPTIONS

There are reasonable arguments for Washington to threaten action in the wake of the Khan Shaykhun attack. The United States has a compelling interest in preventing Assad from using chemical weapons against the Syrian people and in reestablishing the international norm against chemical weapons use that arguably eroded after 2013.

Ideally, the United States could convince Russia to condemn the attack and promise, credibly, to restrain the Assad regime. Unfortunately, this seems unlikely. Russia has made itself central to the Syrian war in part by posing as the regime’s custodian and the guarantor of its good behavior. In practice, however, Russia seems to have limited real influence over the regime, and Moscow is now obfuscating and obstructing multilateral action to avoid exposing its own limitations.

If the United States is going to act unilaterally (or as part of a more limited coalition), anything less than armed coercion is an ineffective distraction.

If the United States is going to act unilaterally (or as part of a more limited coalition), anything less than armed coercion is an ineffective distraction. But any military action should be framed exclusively in terms of chemical weapons deterrence and not in terms of regime change. Washington should link a potential intervention to specific demands that the regime and its allies can understand and reasonably satisfy.

Even that would be a gamble. An intervention that is too low risk—in terms of either means or ends—will not signal resolve or serve as a useful deterrent. And an intervention that seems as though it might be aimed at Assad’s removal will be as threatening to the regime as full military escalation and will thus have no deterrent effect.

U.S. action could also trigger a broader confrontation. Russia will be understandably resistant to allegedly limited U.S. action, given how it was duped in 2011 by an international humanitarian intervention in Libya that ended with the overthrow of the government. For Iran, Syria is an indispensable part of its regional security architecture, and it seems willing to escalate indefinitely if the Assad regime is threatened. Russian and Iranian forces are intermingled with those of the regime, and both countries can retaliate against U.S. interests regionally and internationally.

The United States might be able to mitigate these risks by hitting clearly delineated targets identified in advance, such as chemical weapons sites and airfields, and by explicitly communicating its objectives. But there is no reason to think that Russia and Iran would compliantly evacuate their forces from these targets or that they would take Washington’s stated aims at face value. Whatever U.S. intentions were at the outset, mission creep is real, and miscalculations can happen. And the United States is led by a president who is obviously erratic and dishonest and thus unable to make credible commitments.

DO NO HARM

Yet however the United States chooses to act, it should not try to somehow solve Syria’s war.

Senator Marco Rubio (R-Fla.) and others in Washington who have long been agitating for Assad’s removal have seized on the Khan Shaykhun attack to argue, again, that Assad must go. To be sure, the regime is in fact the main driver of the Syrian war. But if Assad is eliminated, the war won’t end. And from the perspective of the United States in 2017, toppling the dictator is no longer feasible or desirable.

It is a myth that more concerted pressure on the regime will convince it to negotiate a settlement that amounts to its own demise. The Syrian government can be limited geographically—its perimeter can expand or contract—but within those bounds, there is no realistic way to reform it or merge it with appealing elements of the opposition.

What the United States could do is run over both Iran and Russia and, through unilateral military action, gravely weaken or destroy the Assad regime. But if it did, al Qaeda–type jihadists and the Islamic State (also known as ISIS) would be the insurgent forces best positioned to capitalize on the regime's demise.

Tearing down the Assad regime now would throw the most populous parts of Syria into chaos, a vortex of militia violence that would empower jihadists and drive millions of refugees into Syria’s already weakened and destabilized neighbors. If the United States suddenly brought down the regime, it would be impossible to plausibly plan for the day after.

By now, Syria’s war has its own logic and momentum. The United States cannot stop it.

This latest chemical weapons attack arguably means that Washington has an interest in taking more targeted action, both to check Assad’s behavior and to set an example for others, although even that would be complicated and dangerous. But if the United States intervenes now in Syria, it should do so for specific reasons that are consciously separated from the issue of Syria’s broader war. And it needs to make that separation absolutely clear—to Russia, Iran, and the Assad regime, but above all to itself.