BY JACKSON DIEHL

The Washington Post

This presidential election will likely determine whether the United States and Russia undertake a major new reduction of nuclear weapons; whether U.S. arms are supplied to Syrian rebels; whether more U.S. troops are withdrawn from Afghanistan next year; and whether Washington renews pressure on Israel to accept terms for a Palestinian state. It could significantly lower the threshold for a U.S. military strike against Iran.

You wouldn’t know any of that from listening to the conventions, of course. Mitt Romney and Barack Obama appear determined to avoid serious debate. The GOP convention last week echoed with vague slogans about “American leadership” and Obama’s “weakness.” This week, expect to hear lots from Charlotte about the killing of Osama bin Laden and the withdrawal of U.S. troops from Iraq.

That doesn’t mean, as some in the foreign policy world like to argue, that this presidential election won’t change much, even if Romney wins. It’s true that U.S. interests and the pursuit of them tend to remain broadly consistent across presidencies. Obama has fought al-Qaida just as ruthlessly as George W. Bush; if Romney is elected, he will surely drop his threats to start a trade war with China, just as Bush and Bill Clinton did.

There nevertheless are some big and bright differences in this election on foreign policy. More even than those on the economy, they are likely to have practical consequences within months of the election - since, for the most part, action by Congress won’t be necessary. Though the candidates don’t talk about them, they are easy enough to find in their position papers, or in Obama’s case, his first term record.

Start with Russia. Never mind Romney’s much-reported claim that Russia is “our number one geopolitical foe,” or Obama’s oversold “reset” with Moscow. The significant difference is that if Obama is reelected, he will seek to strike a new deal with Vladimir Putin to significantly cut the U.S. and Russian nuclear stockpiles. To do that, he acknowledged last March, he will have to compromise with Putin on U.S. and NATO plans for missile defense; in what he thought was a private aside, he told then-President Dmitry Medvedev that “after my election, I have more flexibility” on that.

Romney’s policy would be close to the opposite. In 2010, he strongly opposed Obama’s New Start treaty with Russia, which made a modest trim in nuclear warheads. Romney meanwhile has promised to boost spending on missile defense, which has been a pet GOP cause for three decades. So there’s one clear choice: less nukes, or more missile defense.

Next come U.S. military engagements, present and potentially future. Both Obama and Romney support NATO’s plan to withdraw combat forces from Afghanistan by the end of 2014, which has provoked some lazy commentary suggesting they don’t differ on the war. In fact, they likely disagree on an urgent question - whether American forces should be reduced next year. Obama is likely to order a cut; Romney has said he will follow the advice of U.S. generals, who will probably recommend that the post-September force of 68,000 be maintained through next year.

In Syria, Obama has repeatedly rejected proposals that the United States help establish safe zones for civilians or supply weapons to the rebels. But Romney has come out for arming the opposition. And what of Iran? Both men have indicated they would use force as a last resort to stop Tehran’s nuclear program. But there is a significant difference: While Obama has said he has “a policy to prevent Iran from obtaining a nuclear weapon,” Romney said in Israel this summer that he would not tolerate an Iranian nuclear “capability.” In other words, Obama probably would use force only if Iran actually tried to build a bomb, while a Romney attack could be triggered if Iran were merely close to acquiring all the means for a weapon - which it is.

Last but not least comes the Israeli-Palestinian conflict. Obama came to office with a burning ambition to broker Palestinian statehood; that and the reduction of nuclear arms seem to be the foreign policy issues that engage him emotionally. The statehood push was one of the administration’s biggest busts, largely because of Obama’s own missteps, and during the election year it has been on hold. Yet it seems likely that a reelected Obama will try again, notwithstanding his poor relations with Israeli Prime Minister Benjamin Netanyahu. Romney, in contrast, has made it clear that he, like George W. Bush in his first term, will put Palestinian statehood on a back burner.

To be sure, these differences may not mean as much to voters as the future of Medicare, or of the Supreme Court. But they do matter - which is why it’s a shame that neither campaign is talking about them.