Tell me something: What do you think would happen if the United States concludes that Iran has been cheating and delaying and is about to pop a fully functional nuclear weapons program? Would President Obama respond by joining Israel to bomb the Iranian nuclear facilities to smithereens — or would he stall and equivocate? My bet is the latter and so, just to double down, is what I bet the Iranians are betting. They have taken the measure of Obama. He lacks menace.

Menace is essential in a world leader if he (or she) is going to be feared as well as admired. Obama falls into the admired category — the leader who won the Nobel Peace Prize with mere good intentions, a guy who had a new attitude toward Russia (a reset) and Iran (an approach) and China (a pivot) and, of course, to the Muslim world — an appreciation from a president who had broken the mold. We know him now as someone miscast: a rational man in an irrational world.

A president’s demeanor is no minor thing. It can matter greatly. For an example, history serves up Gerald Ford, president by virtue of a third-rate burglary. Ford succeeded Richard Nixon, who resigned in 1974 on account of the Watergate scandal. The year before, the U.S. and North Vietnam reached a cease-fire in the Vietnam War. Nixon hailed it as “peace with honor.”

The dishonorable North Vietnamese had other ideas. They not only knew that America was weary of war and that Congress would never reverse the troop withdrawal, but also that Ford was no Nixon. Nixon had cultivated the image of being just a touch mad — what later came to be called his “madman theory” of foreign policy. He wanted America’s adversaries to think he was capable of anything — including the use of nuclear weapons. In 1969, he had even ordered a worldwide nuclear alert. It rattled the Soviet Union. It was a feint, but it made a point. The American president would not be trifled with.

Whether the North Vietnamese believed that Nixon was off his meds is beside the point. What they knew is that a president who might retaliate to an invasion of the South with a bombing campaign had gone into exile. The genial Ford was in the White House. Shucks, he was no bomber. The North marched South, paused to gauge Washington’s reaction, and kept on going. Vietnam then had its version of peace with honor.

Increasingly of late, the bombing campaign against the Islamic State has triggered the nagging finger of Vietnam: here we go again. In a New York Times op-ed piece, Fredrick Logevall and Gordon M. Goldstein warn of the slippery slope. Their article was headlined, “Will Syria Be Obama’s Vietnam?” That about says it all. They remind us that Lyndon Johnson, too, had wanted to avoid boots on the ground — and that events got away from him. Unexpected events is just another way of saying war. Obama knows this and he has insisted that American troops, with the exception of a few advisers and such, will not return to the Middle East.

I take the Vietnam analogy/precedent seriously. But I also take seriously how one of those uncontrollable events was Nixon’s replacement by Ford. And I take even more seriously the Iranian nuclear program and Iran’s apparent foot-dragging. Will they be deterred by Obama? Did they notice how he called for Bashar al-Assad to go, but left it at that? Did they notice that the president refused to aid the Syrian rebels until things really got out of hand? Did they notice how Syria blew through his red line and the president of the United States did nothing? You bet they did. What they saw was weakness, a president so resolutely determined to avoid the mistakes of the past that he was making new ones of his own.

There are two Vietnam precedents. One is about escalation, the other about perceived weakness. Obama has to avoid both. At the moment, he has to be resolute in his campaign to eliminate — as far as possible — the Islamic State and not conduct the air campaign in a dilatory manner. He has gone to war — an air war, to be sure, but a war nonetheless. It must be pressed relentlessly, and not just to contain the Islamic State until other Arabs can eliminate it, but to convince Iran (and others) that he means what he says. This is one event he can control.

Richard Cohen is a writer with the Washington Post Writers Group. He can be reached at cohenr@washpost.com.