Last weekend, Texas Gov. Rick Perry accused Sen. Rand Paul of being too much of an "isolationist." This week, Paul fired back, asking Perry how many American kids should die for an Iraq that Iraqis won't or can't defend themselves.

It's gratifying to see two potential 2016 Republican candidates fighting about foreign policy, because foreign policy is one of the areas where American presidents still have relatively free rein to pursue their own course. That means these barbed speeches and press releases may constitute the most important debate of the nascent 2016 presidential election.

One of the faults of the American system of governance is that the public tends to elect presidents based on feelings about the economy. Every single poll from Gallup leading into 2012 showed that voters listed the economy as their top issue.

But the truth is that presidents can do relatively little to improve the economy. Whether they want a major new executive branch program, a round of stimulus spending, or revisions to the tax code, they have to go through Congress, a body that is in the habit of resisting large-scale transformations of the American state. Usually less than 5 percent of voters claim foreign policy is a paramount issue (although some 30 percent or more will say "terrorism" is on their minds).

But foreign policy is what presidents can do. A combination of congressional lassitude, partisan docility, and changes in how military force is authorized has given the president broad power to send bombs, armed drones, military advisers, and even soldiers around the planet in search of monsters to destroy.

Remember the big debate in 2011 about using the U.S. military to bomb Libya and help rebels effect regime change there? Of course you don't. There wasn't one. Operation Odyssey Dawn was something that Barack Obama just did. It proceeded with all the public deliberation you might take for calling in sick on a Friday and driving to a lake house: none. And the results have been less impressive than he hoped.

In the 1990s, President Clinton began bombing Serbia hours after the Senate passed a non-binding authorization of force endorsing the idea. The House would take several votes on the conflict almost a month later, with lawmakers still undecided on whether to authorize more force, to go along with the Senate, or to pull out altogether. The bombs kept falling anyway. Clinton also lobbed a few missiles into Sudan without consulting Congress.

After 9/11, Congress basically handed the Bush administration a blank check to use force anywhere against anyone who could plausibly be put in a terrorist database. Our drone program has expanded beyond Afghanistan and Iraq to include Pakistan, Yemen, and Somalia, so far.

All this is to say that it's an eminently good thing that at least seven Republicans have come out to argue with Rand Paul about foreign policy. Custom, history, and the remains of the Constitution have arranged the American polity in such a way that it is on this issues of diplomacy and war that presidents make a real difference.

We'll surely spend too much time on gimmicky 59-point plans for the economy between now and November 2016. But those ambitions will be chewed up like cud on Capitol Hill. Instead, we need to find out how candidates conceive of America's role in the world, how they perceive America's enemies, how broadly they define the American interest, and how willing and anxious they are to send our sons and daughters to die for a cause.