The clearest difference between the two candidates may be on the issue that’s most dangerous: Iran.

Obama has demanded that Iran halt its nuclear program, and has warned that if Iran acquires a nuclear weapon, the U.S. might respond with a military attack.

Romney has gone further, suggesting that he might launch a preemptive military strike even before Iran acquires a nuclear weapon — when the Tehran regime reaches “nuclear capability” — an earlier, less clearly defined status.

Romney and his aides have refused to spell out what they mean by “nuclear capability.” And Romney muddied the waters last week when he told ABC News that his “red line” on Iran was the same as Obama’s.

Romney has also hinted that he thinks the time for negotiations with Iran is over. “We’re still talking and Iran’s centrifuges are still spinning,” he complained in his acceptance speech at the Republican National Convention.

Essentially, Romney’s position is that a clearer, earlier threat of U.S. military intervention is more likely to compel Iran to yield than Obama’s more ambiguous warnings. He may be right, but if he turns out to be wrong, his position would drive the United States into war more quickly than Obama’s, with fewer chances to find a third way out.

There will be tumult in the Middle East for years to come, no matter which candidate is elected this fall; Islamist militants don’t tailor their tactics according to which party holds the White House.

In November, voters won’t only be choosing between candidates who want to raise taxes or lower them on upper-income families, maintain domestic spending or cut it, keep a healthcare law or repeal it. They’ll be choosing, as well, between a president who wants to continue a cautious, thrifty policy of alliance-building overseas — and one who rejects that “risk-averse” approach for a costlier, but bolder, U.S. role.