1. Congress’s partisan makeup largely determines whether it goes along with the president on military action.

This is pretty obvious, but it speaks to the theory that the Republican-controlled House is much less likely to approve a military authorization on Syria. Writing in Foreign Affairs, William Howell and Jon Pevehouse argue:

When the opposition party holds a large number of seats or controls one or both chambers of Congress, members routinely challenge the president and step up oversight of foreign conflicts; when the legislative branch is dominated by the president’s party, it generally goes along with the White House.

The authors say it was this partisan consensus, rather than some sort of collective congressional inattentiveness, that explains why lawmakers were so supportive of George W. Bush’s Iraq operations: “Partisan unity, not institutional laziness, explains why the Bush administration's Iraq policy received such a favorable hearing in Congress from 2000 to 2006.”

But beyond just the votes, it turns out party control also determines how many oversight hearings are held to hash out the potential consequences of military action in the first place. “... During the post-World War II era, when the same party controlled both Congress and the presidency, the number of hearings about military policy decreased, but when the opposition party controlled at least one chamber of Congress, hearings occurred with greater frequency.”

In this case, Obama’s aggressive Syria lobbying campaign might partly be necessary because of the partisan opposition he faces in the House.

2. Everyday people tend to support waging war when political elites seem eager to do so.

Just 20 percent of Americans in a recent Reuters/Ipsos poll said the United States should take action in Syria. Meanwhile, 76 percent of Americans supported the Iraq War before it began. Though one would hope that we’re basing these opinions on a careful calculation of the relative costs and benefits of intervention, that’s not really true, according to MIT political scientist Adam J. Berinsky.

In a 2007 Journal of Politics article, Berinsky argues that rather than reading history books, articles on military theory, and/or Atlantic stories, Americans base our support for foreign wars on whether the policymakers we like want to intervene.

Looking at both World War II and the recent Iraq War, Berinksy found that citizens at the time had little knowledge about the particulars of the conflicts -- such as Nazi atrocities or American troop casualties -- and thus could not have determined the relative advantages or disadvantages of America’s actions there. Instead, people relied on cues from lawmakers and pundits, whose own, party-based beliefs in turn shaped the public’s.

In fact, Berinsky found that even as people become more informed about the Iraq War, they paradoxically became both far more AND less likely to support the intervention -- depending on their political persuasion: