About midway through the Democratic debate, the previously staid affair erupted. Bernie Sanders lit into Hillary Clinton on an issue where there are real differences between the two candidates — launching wars to topple foreign dictators.

The flashpoint was Bashar al-Assad. Sanders argued that the United States should abandon any commitment to toppling Assad's government, and focus on destroying ISIS instead. "I worry," Sanders said, "that Secretary Clinton is too much into regime change:"

Yes, we could get rid of Saddam Hussein, but that destabilized the entire region. Yes, we could get rid of Qaddafi, a terrible dictator, but that created a vacuum for ISIS. Yes, we could get rid of Assad tomorrow, but that would create another political vacuum that would benefit ISIS. So I think yeah, regime change is easy, getting rid of dictators is easy, but before you do that you've got to think about what happens the day after. And in my view what we need to do is put together broad coalitions to understand that we're not going to have a political vacuum filled by terrorists, that in fact we are going to move steadily and maybe slowly toward democratic societies. In terms of Assad, [he's] a terrible dictator, but I think in Syria the primary focus now must be destroying ISIS and working over the years to get rid of Assad. That's the secondary issue.

"That is exactly what I just said and what I just described," Clinton fired back. She then noted that Sanders, too, supported toppling Qaddafi ("you voted for regime change with respect to Libya") but then also said that she thinks ignoring Assad would actually hurt the fight against ISIS:

We will not get the support on the ground in Syria to dislodge ISIS if the fighters there who are not associated with ISIS but whose principal goal of getting rid of Assad don't believe there is a political, diplomatic channel that is ongoing. We now have that we have the U.N. Security Council adopting a resolution that lays out a transition path. It's very important we operate on both at the same time. And let me just say a word about coalition building because I've heard senator Sanders say this. I know how hard it is to build coalitions. I think it would be a grave mistake to ask for any more Iranian troops inside Syria. That is like asking the arsonist to come and pour more gas on the fire. The Iranians getting more of a presence in Syria, linking with Hezbollah, their proxy in Lebanon, would threaten Israel and would make it more difficult for us to move on a path to have a transition that at some point would deal with Assad's future.

This was the most substantive and, frankly, emotional exchange on foreign policy between the two leading Democrats throughout the campaign. It speaks to a remaining, if understated, divide between the Democratic party's buoyed post-Iraq dovish faction and long-dominant liberal hawks."