For all the apology tour nonsense and despite the bitter squabbling over the attacks in Libya, it’s actually been a bit hard to figure out where Mitt Romney actually disagrees with President Obama on foreign policy.

So far, he’s said he agrees with the 2014 withdrawal in Afghanistan. He agrees with the sanctions in Iran. He doesn’t want to restart the Iraq War. It’s not clear what he’d do in Libya or Syria. He wants to call China a currency manipulator, but after that, who knows? He has not criticized Obama's use of drones, or his incursions into Pakistan to kill high-value targets like Osama bin Laden.

There’s one exception though. Defense spending. Here Romney’s been pretty specific. It’s right there on his Web site. He wants, he says, to give defense spending “a floor of 4 percent of GDP.” Compared to Obama’s proposals, and the military’s current requests, that's an increase of $2 trillion over the next decade. That’s a huge amount of money.

Up With Chris/Foreign Policy

Romney’s not said exactly what he wants to do with it. He wants to add 100,000 soldiers and accelerate shipbuilding, but that doesn’t come to $2 trillion. So aside from more money on defense, it’s a bit hard to say what his vision for the military is. "The plan proceeds from a strategic worldview that more is better," says Heather Hurlburt, director of the National Security Network. "But if you want to get more detailed about what they'd do with those troops, they just haven't really said. You can't say they subscribe to doctrine X that was elucidated by admiral Y in 1952. There’s nothing like that."

It got even harder this week. According to Bloomberg News, Dov Zakheim, a former Pentagon comptroller who advises Romney, said, quote,

“The goal of 4 percent of GDP remains and is unchanged. But that goal is not going to be achieved overnight or perhaps even by the end of the first term.”

This is a common issue with the Romney campaign: The more you hone in on specifics, the more they begin to protest and say you can’t really read the numbers in a straightforward way. That way, they can have the applause line, but they can't be attacked for the policy's downside.

That said, what seems clear is that one difference between the candidates is that Romney intends to spend much, much more on defense than Obama, which will mean cutting much deeper into other programs. What we should be looking to hear tonight is a clearer answer on what, exactly, that extra money would be used for.