Iraq is basically falling to pieces. The ultramilitant Sunni group the Islamic State of Iraq and Syria (ISIS) has captured Mosul after total desertion by whole divisions of the Iraqi army, and is pressing toward Baghdad. An independent Kurdistan has become a distinct possibility, with Kurdish forces having captured the northern city of Kirkuk.

This is a horrifying development, and our country is largely responsible for this disaster. A vigorous debate is emerging over how the Obama administration should respond. But the key question for Americans is how to prevent the political class from luring the country into another pointless war down the line.

It was shades of 2002 this week, when Kenneth Pollack — author of the wretched book The Threatening Storm, which convinced many liberals to support the Iraq invasion — published an op-ed in the Wall Street Journal arguing that we need to bomb Iraq in part to secure its oil supply.

Pollack is always down for dropping ordnance on someone, but we know less about the thoughts of Hillary Clinton, who has been consistently hawkish for most of her career. As I have repeatedly pointed out, she would be president today if she hadn't supported the 2003 Iraq invasion. But according to her new memoir, she seemingly hasn't changed much — she strongly supported David Petraeus' plan to arm Syrian rebels, and firmly fixed blame for killing the plan on President Obama.

This is why we need a challenger to Clinton's left on foreign policy. Like Conor Friedersdorf, I think Russ Feingold, the former senator from Wisconsin and a bona fide anti-war liberal, would make an excellent candidate.

To be sure, there is a strong chance that Clinton will be our next president. She is currently leading the Democratic primary polls by something like 50 percentage points. Feingold would almost certainly lose.

But winning wouldn't be the point — the point would be to make Clinton worry about her left flank. Though she seems to be a true hawk, she surely realizes that Obama's opposition to the Iraq War was his most important advantage in 2008. All Feingold would have to do is mount a credible enough challenge to get Clinton to promise not to invade random countries for no reason.

Clinton may well reverse those promises the moment she takes office — though contrary to popular belief, typically presidents do try to keep their promises. And voters would never let her forget it if she went back on her word.

There's another reason Feingold should run, as opposed to another lefty like Elizabeth Warren, who is currently sitting in Congress as the junior senator from Massachusetts. Congress has essentially abdicated its oversight role over foreign policy, which means presidents get a largely free hand in foreign affairs. Any strategy to rein in the executive must rely on a two-pronged approach that places equal or greater emphasis on the legislative branch, as compared to electing the right person. Warren ought to stay in the Senate where she's much more needed.

The chances of a new full-blown American invasion of Iraq are quite low. The political winds are very, very different than in 2002. But keeping the war machine in its cage is a constant struggle, and it will be harder as the memory of the disastrous Iraq invasion fades.