Americans who watched Monday's debate learned that Mitt Romney wanted to sound like Barack Obama on major world issues and that Obama wanted to highlight the contrasts. They learned that Romney can be vague and Obama can be condescending. They learned that Bob Schieffer can't keep a discussion from veering off course.

What they didn't learn is what the next president's foreign policy will look like or whether to expect war or peace. That's not the fault of the debate. It's the fault of the candidates—not just these two, but pretty much all presidential candidates. Ultimately, it's the fault of voters, who repeatedly give their presidents a blank check.

Romney was intent on fuzzing the picture of how he will confront international problems. Having waxed hawkish in the primaries to charm Republican voters, he felt the need to show a gentler side to independents.

He said the administration's sanctions on Iran are working. He rejected using the U.S. military in Syria. He promised to withdraw from Afghanistan by the end of 2014. Referring to terrorism, he said, "We can't kill our way out of this mess." Numerous sentences could have been borrowed from Obama, circa 2008.

Obama, however, painted Romney as trigger-happy and unreliable, bouncing irresponsibly from one position to another on Afghanistan, Iraq and Libya. The president promised Iran will not get nukes. He bragged about toppling Moammar Gadhafi and wrapped himself in the Israeli flag. He wanted everyone to know he'll be smarter and tougher than Romney.

Maybe so. But if we've learned anything from past presidential campaigns, it's that we never know what the winner will do when it comes to international affairs. Some candidates won't say what they plan to do with the U.S. military, and some of them don't know.

In 1964, Lyndon Johnson publicly rejected a bigger U.S. role in Vietnam, saying, "We are not about to send American boys nine or ten thousand miles away from home to do what Asian boys ought to be doing for themselves"—only to plunge into war after defeating Barry Goldwater.

Ronald Reagan campaigned against nuclear arms control as a candidate, but as president, he proposed a deal with the Soviets to eliminate such weapons completely. Bill Clinton faulted George H.W. Bush for refusing to intervene in the Balkan wars—only to stall for years before intervening himself.

George W. Bush criticized Clinton for overextending the American military and bogging us down in nation-building abroad. He then spent his presidency doing the same things on a much bigger, bloodier scale.

When Obama was reminding voters that he had opposed the Iraq invasion, no one dreamed he would use force to achieve regime change in yet another Arab country that lacked nuclear weapons and posed no threat to us: Libya. Nor did voters anticipate Obama's drone wars in Yemen and Somalia.

We should expect more surprises in the next four years. Both candidates oppose using force in Syria right now, but if the carnage grows larger and more visible, the president could very well deploy bombers or even ground troops.

Afghanistan? They agree now on pulling out, but if the security environment deteriorates and the Afghan army proves insufficient to the task—both likely—all bets are off. Chinese and U.S. differences on Taiwan and the South China Sea could suddenly turn into a crisis that leads to shots being fired.

There is also the minor matter of Iran. Both Obama and Romney have indicated they will use force if necessary to stop its nuclear weapons quest. If Iran continues in that direction, Americans may abruptly find themselves in another major Middle Eastern war.

Voters may not like that idea, but the next president won't give great weight to public opinion because he won't need to. Americans rarely hold their leaders accountable for unforeseen, unnecessary wars that cost much and achieve little.

In November 2004, Iraq was clearly a disaster, but Bush won reelection. Clinton paid no price for the "Black Hawk Down" debacle in Mogadishu, Somalia. Reagan won by a landslide a year after terrorists blew up 241 U.S. troops in Lebanon, prompting our departure.

Each time, Americans were surprised. Each time, they were unhappy. But each time, they gave the president a pass. You think Obama and Romney didn't notice?