John Allen Gay notices  that liberal interventionists are not very vocal at the moment:
As the civil war in Syria drags on and extremists kill and pillage in northern Mali, one would expect regular and loud cries for the United States to do something to protect innocents. We are now in the fifth year of a Democratic administration, which in theory should be more receptive to this line of thinking. Yet there is little such talk. Where have all the liberal interventionists gone? Have they reformed? Are they in intellectual exile?
Liberal interventionists haven’t gone anywhere, and I don’t think any have changed their views all that much in recent years. What has happened is that liberal interventionists successfully pushed for one major military campaign in Obama’s first term, and now there is not much appetite for any more military interventions in the near future. In the case of Syria in particular, many of the Libyan war’s supporters have not endorsed military action in Syria and some have become skeptics of pro-intervention arguments.
There are still some vociferous advocates for a more aggressive and activist Syria policy  but there is nowhere near the same degree of intensity or breadth of support that existed two years ago. This is because some liberal interventionists recognize correctly that intervening militarily in Syria would much more dangerous, contentious, and counterproductive than the war in Libya was, as well as being illegal. Others may not want to apply the Libya and Kosovo precedents to the Syrian case because they don’t want to be perceived as knee-jerk proponents of new wars. It’s likely that the election campaign and the administration’s reluctance to take a more activist position also account for some of what Gay observes. While there was significant support inside the administration to intervene in Libya, that is noticeably lacking this time around, so some liberal interventionists may not be interested in berating Obama for “inaction” when he is already coming under attack from Republican hawks for the same thing. In this case, partisan loyalty might actually be blunting interventionist impulses rather than encouraging them. The memory of the Iraq war remains a powerful obstacle to any new war in Syria, but I suspect that the realities of the Syrian conflict are having a far more significant influence in discouraging support for a larger American role there.