One point that Rand Paul constantly makes (see this Wall Street Journal op-ed for instance) is that we wouldn’t want to undermine Bashar Assad because he’s such a bulwark against the Islamic State. The problem with this is that Assad in recent years, in large part, hasn’t fought the Islamic State and indeed, allied himself with it during the Iraq War. Fred Hof recounts some of the details in this piece at The New Republic:

The Assad regime initially escorted Al Qaeda in Iraq operatives from Syria to Iraq between 2003 and 2011, but its violently sectarian response to peaceful protest drew much of what was left of the seemingly beaten Al Qaeda in Iraq back to Syria, where it was joined by foreign fighters and split into two groups: the Islamic State and the Nusra Front. Both groups compete with the nationalist opposition to Assad—indeed, the Islamic State engages in de facto collaboration with the regime in western Syria to erase the nationalists, even as Assad and the caliph clash in eastern Syria over oil fields and air bases. And it was from secure bases in eastern Syria that the Islamic State launched its recent assault into Iraq . . .