As the Obama administration considers military action in Syria, Rosie Gray wonders why the antiwar movement has been strangely muted: "Though some groups have organized online petitions and some real-life protests," she reports, "the antiwar crowd that was on fire before the war in Iraq has made hardly a dent in the conversation surrounding Syria."

So where did the protesters go? One obvious theory is that Syria is just vastly different from Iraq — no one's talking about sending in ground troops this time. That could explain why the opposition isn't quite so fierce.

Alternatively: It's also only been a few days since U.S. officials have started talking seriously about lobbing missiles at targets in Syria. Maybe the protests just haven't had time to gather steam yet. Wait and see.

A third possibility, though, is that the election of Barack Obama sapped the energy of the U.S. antiwar movement. Reihan Salam points to a 2011 paper by sociologists Michael T. Heaney and Fabio Rojas, who find that antiwar protests shrunk very quickly after Obama took office in 2008 — mainly because Democrats were less likely to show up:

Drawing upon 5,398 surveys of demonstrators at antiwar protests, interviews with movement leaders, and ethnographic observation, this article argues that the antiwar movement demobilized as Democrats, who had been motivated to participate by anti-Republican sentiments, withdrew from antiwar protests when the Democratic Party achieved electoral success, if not policy success in ending the wars in Iraq and Afghanistan.

Heaney and Rojas begin by puzzling over a paradox. Obama ran as an antiwar candidate, but his first few years in office were rather different: "As president, Obama maintained the occupation of Iraq and escalated the war in Afghanistan. The antiwar movement should have been furious at Obama’s 'betrayal' and reinvigorated its protest activity. Instead, attendance at antiwar rallies declined precipitously and financial resources available to the movement dissipated."

But even though the wars were far from ended, the two researchers find that the size of antiwar protests rapidly dwindled between 2007 and 2009 anyway:

One reason for that? Democrats themselves stopped showing up for protests, with only a small rebound in December 2009 after Obama announced that he would send 30,000 additional troops to Afghanistan:

"Since Democrats are more numerous in the population at large than are members of third parties, the withdrawal of Democrats from the movement in 2009 appears to be a significant explanation for the falling size of antiwar protests," Heaney and Rojas write.

That theory jibes with some of Gray's reporting. "The Democrats are missing in action because of course the president is a Democrat," one longtime activist tells her. "That’s the biggest factor, I think. What’s tamping down the activism is partisanship." Still, that's not the only theory out there — the poor economy is another reason why nonprofit groups might be struggling — and Gray's piece is worth reading in full.