Through 11 presidential elections, beginning with the Democrats’ nomination of George McGovern in 1972, Republicans have enjoyed a presumption of superiority regarding national security. This year, however, events and their rhetoric are dissipating their advantage.

Hours — not months, not weeks, hours — after the last U.S. troops left Iraq, vicious political factionalism and sectarian violence intensified. Many Republicans say Barack Obama’s withdrawal — accompanied by his administration’s foolish praise of Iraq’s “stability” — has jeopardized what has been achieved there. But if it cannot survive a sunrise without fraying, how much of an achievement was it?

Few things so embitter a nation as squandered valor; hence Americans, with much valor spent there, want Iraq to master its fissures. But with America in the second decade of its longest war, the probable Republican nominee is promising to extend it indefinitely.

Mitt Romney opposes negotiations with the Taliban while they “are killing our soldiers.” Which means: No negotiations until the war ends, when there will be nothing about which to negotiate. “We don’t,” he says, “negotiate from a position of weakness as we are pulling our troops out.” That would mean stopping the drawdown of U.S. forces — except Romney would not negotiate even from a position of strength: “We should not negotiate with the Taliban. We should defeat the Taliban.” How could that be achieved in a second decade of war? What metrics would establish “defeat”? Details to come, perhaps.

The U.S. defense budget is about 43 percent of the world’s total military spending — more than the combined defense spending of the next 17 nations, many of which are U.S. allies. Are Republicans really going to warn voters that America will be imperiled if the defense budget is cut 8 percent from projections over the next decade? In 2017, defense spending would still be more than that of the next 10 countries combined.

Do Republicans think it is premature to withdraw as many as 7,000 troops from Europe two decades after the Soviet Union’s death? About 73,000 will remain, most of them in prosperous, pacific, largely unarmed and utterly unthreatened Germany. Why do so many remain?

Since 2001, the United States has waged war in three nations, and some Republicans appear ready to bring the total to five, adding Iran and Syria. (The Weekly Standard, of neoconservative bent, regrets that Obama “is reluctant to intervene to oust Iran’s closest ally, Syrian President Bashar al-Assad.”) GOP critics say that Obama’s proposed defense cuts will limit America’s ability to engage in troop-intensive nation-building. Most Americans probably say: Good.

Critics say that defense cuts will limit America’s ability to intervene abroad as it has recently done. Well. Even leaving aside Iraq and Afghanistan, do Americans want defense spending to enable a rump of NATO — principally, Britain and France — to indulge moral ambitions and imperial nostalgia in Libya, and perhaps elsewhere, using U.S. materiel and competence?

Defense Secretary Leon Panetta says that the Army should contract from 570,000 soldiers to 490,000 in a decade. Romney says that the military should have 100,000 more troops than it does. (The Army is 88,000 larger than it was before Afghanistan and Iraq.) Romney may be right, but he should connect that judgment to specific assessments of threats and ambitions.

Romney says: “It is unacceptable for Iran to have a nuclear weapon,” that if he is elected, Iran will not get such a weapon, and if Obama is reelected, it will. He also says that Obama “has made it very clear that he’s not willing to do those things necessary to get Iran to be dissuaded from” its nuclear ambitions.” Romney may, however, be premature in assuming the futility of new sanctions the Obama administration is orchestrating, and Panetta says Iran acquiring nuclear weapons is “unacceptable” and “a red line for us” and if “we get intelligence that they are proceeding with developing a nuclear weapon, then we will take whatever steps necessary to stop it.” What, then, is the difference between Romney and Obama regarding Iran?

Osama bin Laden and many other “high-value targets” are dead, the drone war is being waged more vigorously than ever, and Guantanamo is still open, so Republicans can hardly say that Obama has implemented dramatic and dangerous discontinuities regarding counterterrorism. Obama says that, even with his proposed cuts, the defense budget would increase at about the rate of inflation through the next decade. Republicans who think America is being endangered by “appeasement” and military parsimony have worked that pedal on their organ quite enough.

georgewill@washpost.com