My colleague Kelly Cohen flagged a Wall Street Journal op-ed by Sen. Rand Paul, R-Ky., making the case for how U.S. interventionists helped foster the growth of ISIS.

Most of Paul's argument focuses on how ISIS has benefitted or could benefit from certain U.S. actions, but the broader issue he raises is that American leaders should look to past mistakes to guide future decisions.

It's one thing for Paul to make this argument as a blogger, or as the jumping off point for articulating an actual policy.

But the problem is that Paul is writing this op-ed as a prospective presidential candidate. As president, he would have to deal with threats such as the one posed by ISIS. And he won't be able to go back in time to reverse foreign policy decisions. He will have to grapple with the world as it is. It won't be enough to offer a critique, he will have to set actual policy. Yet he offers no window into how he might address a problem such as ISIS.

"Only after recognizing the practical limits of our foreign policy can we pursue policies that are in the best interest of the U.S.," he wrote. "The Islamic State represents a threat that should be taken seriously. But we should also recall how recent foreign-policy decisions have helped these extremists so that we don't make the same mistake of potentially aiding our enemies again."

There's no indication of what it would mean to make sure ISIS is "taken seriously" within his non-interventionist foreign policy framework.

As my colleague Becket Adams explored, the winning Republican candidate will likely have to thread the needle between responding to the conservative base's disgust with what they see as President Obama's weakness and absenteeism on foreign policy, while also speaking to those who are increasingly skeptical of military action in the wake of the failures of the Iraq War.

To be fair, it isn't the presidential campaign yet, and Paul has time to lay out a more detailed foreign policy. If anything, at this stage, Paul's foreign policy ideas are more fleshed out than a lot of other possible Republican candidates, especially the governors.

But Paul faces a unique challenge of trying to apply his non-interventionist ideas to current problems. And when he's facing a Republican electorate that is increasingly angry at Obama for being apathetic in the face of emerging threats, selling an even more hands-off approach could be an insurmountable obstacle.

If Paul, like his father, former Rep. Ron Paul, merely wants to launch a presidential candidacy to give voice to his brand of Republicanism, that would be one thing. But if he wants to launch a serious campaign that he intends to win, he'll have to be able to articulate what he would do about actual problems that he'd inherit as president — not what he would have done differently if he possessed a time machine.