Politics stops at the water's edge? Not recently.

Robert J. Lieber is professor of government and international affairs at Georgetown University. His latest book is Power and Willpower in the American Future: Why the United States is Not Destined to Decline.
Does polarization in domestic politics affect foreign policy as well? There is a long-standing belief that it should not. A classic statement of that view can be found in the widely cited words of a leading Republican senator in the early days of the Cold War. Speaking in 1947, Sen. Arthur Vandenberg (Mich.), the influential chairman of the Foreign Relations Committee, provided key support to Democratic President Harry S. Truman and admonished his colleagues that “we must stop partisan politics at the water’s edge.”  And in practice, when it comes to military intervention, both the urgency of events and rally-round-the-flag effects are often conducive to wider support within Congress and among the general public. Thus Gallup has found that in 10 conflicts over the past two decades, initial public approval averaged 68 percent.
But, with the passage of time, and especially if results fall short of initial expectations, party differences over foreign policy tend to widen, both because of disagreements over the issues at stake and as a result of elite leadership. As evidence of this, those who identify as Democrats are much more inclined to support the foreign policies of a Democratic president, and Republicans are likely to take cues from a Republican occupying the White House. In today’s polarized climate, politics can be delayed at the water’s edge, but it certainly doesn’t stop for long.
Iraq provides a dramatic case in point. At first the intervention received broad support. On Oct. 10-11, 2002, the Senate and the House passed resolutions authorizing President George W. Bush to use armed force in Iraq. The measure received overwhelming GOP backing, although a majority of Senate Democrats also voted in favor (29-21), with most presidential aspirants (Sens. Hillary Rodham Clinton, Joseph R. Biden Jr. and John F. Kerry) voting yes. There was stronger Democratic opposition in the House, but nearly 40 percent of House Democrats did support the resolution. Public opinion was favorable as well. Less than a month before the outbreak of war, a Gallup poll found 59 percent of the public supporting military action, and shortly after the start of the conflict on March 20, 2003, a Pew poll found 72 percent of the public describing the use of force as the right decision.
With time, rising casualties, no clear end in sight, and sharply polarized views of President George W. Bush, opinion about the Iraq war shifted, becoming less favorable and increasingly polarized. The extent of partisan differences was stunningly apparent in a New York Times-CBS poll of delegates to the 2008 Democratic and Republican national conventions. On the question of whether the United States “did the right thing in taking military action against Iraq,” 80 percent of Republican delegates and 70 percent of Republican voters agreed. In stark contrast, only 14 percent of Democratic voters responded positively and just 2 percent of Democratic delegates did.
These figures document not only wide public disagreement, but an even more stunning gap among political elites. Moreover, the near-complete absence of support for the Iraq war among Democratic delegates suggests the demise of the moderate to conservative current within the Democratic Party as exemplified by the political marginalization and retirement of former senator Joseph I. Lieberman (I-Conn.). Whether the result of beliefs, war weariness or partisan sorting, these figures suggest that in foreign policy at least, political polarization may be reasonably symmetric. That stands in contrast to the recent APSA report’s conclusion that — in the words of Nolan McCarty — “despite the widespread belief that both parties have moved to the extremes, the movement of the Republican Party to the right accounts for most of the divergence between the two parties.”
Since the withdrawal of American troops in 2011, Iraq has been in the headlines far less frequently. Syria is, however, and last autumn, partisan differences became evident over the possible U.S. use of force there. This time, party positions on intervention were reversed, although the differences have been less dramatic than over Iraq. In a Gallup poll taken Sept. 3-4, 2013, at a time when President Obama appeared to be proposing action, Republicans opposed a military strike (31 percent in favor, 58 percent against). Their responses were close to those of the public as a whole (36 percent in favor, 51 percent against), while Democrats remained divided (45 percent vs. 43 percent).  On the whole, these results reflect a reaction to 12 years of military involvement in the region as well as the consequences and costs, human and material, of those commitments. At the same time, the differences between Democrats and Republicans are influenced by the fact that Obama rather than George W. Bush occupies the White House.
Over the decades, support for foreign military intervention has tended to be cyclical. Public support often declines after wars, as it did following World War I, World War II, Korea and Vietnam, and as now appears to be the case in regard to Afghanistan, Iraq and Libya. Indeed, Afghanistan is considered the most unpopular of all recent American conflicts. A late December CNN/Opinion Research poll found public support for that war having dropped to 17 percent, a figure lower even than for Iraq and Vietnam at their most unpopular moments
Nonetheless, and as in the past, foreign policy elites do remain more supportive of the United States continuing to play an active role in world affairs than does the public at large. A major opinion study by the Pew Research Center for the People and the Press in collaboration with the Council on Foreign Relations (CFR), found 51 percent of the public taking the view that the United States does too much in helping solve world problems, while just 17 percent saw involvement as too little. By contrast, CFR respondents (a group encompassing foreign policy experts and practitioners) gave virtually opposite answers, with 21 percent saying too much and 41 percent too little.  Eventually, as the memory of recent conflicts recedes, and with new threats and challenges, public opinion will evolve yet again. At the same time, partisan loyalties and presidential politics will continue to manifest themselves. In short, politics does not stop at the water’s edge.
Further Reading
This is the latest post in our ongoing series on political polarization. The previous posts are listed below.  -Dan Hopkins
What we do know and don’t know about our polarized politics
American politics is more competitive than ever. That’s making partisanship worse.
Polarization we can live with. Partisan warfare is the problem.
How political polarization creates stalemate and undermines lawmaking
Electing more women to Congress isn’t a solution for polarization
How U.S. state legislatures are polarized and getting more polarized (in 2 graphs)
How ideological activists constructed our polarized politics
Our politics may be polarized. But that’s nothing new.
Our politics is polarized on more issues than ever before
How race and religion have polarized American voters
Can young voters break the cycle of polarization?
Americans aren’t polarized, just better sorted
How politically moderate are Americans?  Less than it seems.
The real extremists are American voters, not politicians
How better educated whites are driving polarization
How our partisan loyalties are driving polarization
Party polarization is making is more prejudiced
Why don’t Americans trust the government? Because the other party is in power.
Are Fox and MSNBC polarizing America?
Why you shouldn’t blame polarization on partisan news.
The media makes us think we’re more polarized than we are.
How to fix our polarized politics?  Strengthen political parties.