By Bruce Stokes, Director of Global Economic Attitudes, Pew Research Center

President Barack Obama arrived today in St. Petersburg, Russia, for the G-20 Summit, the annual get-together of the world’s big powers that is likely to be dominated by discussion of Syria. In his bilateral discussions with other world leaders, Obama will be pressing for their support for his proposed military action against Syria’s chemical weapons capability. But his challenge may be less with heads of state than it is with their populations, including his own.

If countries held referendums before they engaged in military action, there would be far fewer wars. But the decision to use military force is a strategic one, not an exercise in participatory democracy. For that the Obama administration should be thankful. In the run up to the congressional vote on a U.S. military strike against Syria , the American people stand strongly opposed to any such action, as are people in most other parts of the world.

So the White House faces a formidable challenge: turn around deep-seated public opposition both at home and abroad to military involvement in the Syrian civil war or take such action without public backing. But history offers some reassurance for the administration: the task gets easier if and when a military strike is actually launched. Publics almost everywhere tend to rally round the flag, at least in the initial phase of any combat operation. But public opinion polling also contains a warning: as a conflict drags on, support generally wanes.

By a 48 percent to 29 percent margin, more Americans oppose than support conducting military airstrikes against Syria, according to a new telephone survey by the Pew Research Center, conducted Aug. 29-Sept. 1. The survey was conducted after widespread press coverage of allegations that the Syrian government had employed chemical weapons. By an even wider margin of 59 percent to 28 percent a majority of Americans think that the United States should first get a United Nations resolution authorizing the use of force before taking military action against Syria.

This public reluctance to intervene is widely shared by America’s allies and by people in the Middle East.

In Britain, an Aug. 30-Sept. 1 telephone survey by the BBC found that by more than three-to-one (71 percent to 20 percent) the British felt that Parliament had done the right thing in stopping Prime Minister David Cameron’s government from participating in an international military response to the alleged use of chemical weapons in Syria. In France, 59 percent are against French military engagement in Syria, even if it has U.N. sanction, according to an Aug. 26-28 IFOP online survey. In Germany, a majority of Germans (58 percent) oppose a Western military intervention in Syria, according to an Aug. 28 telephone survey by the Mannheim Research Group. Just a third of Germans (33 percent) approve of it.

Earlier surveys among Syria’s neighbors found similar opposition to Western involvement. Some 80 percent of Lebanese and 68 percent of Turks were against Western countries even sending arms and military supplies to anti-government groups in Syria, according to a March 2013 Pew Research Center poll. Only a slim majority of Jordanians (53 percent) backed such an effort.

And obtaining U.N. authorization for military action may not provide much multilateral political cover for what would be effectively a unilateral strike by the United States. A 2011 Pew Research Center survey of 23 countries found that in only nine of these nations did a majority or plurality of the public say U.N. approval was needed to deal with international threats. In six countries, majorities or pluralities thought seeking approval was not necessary. Publics were divided in the eight other nations.

Public reluctance to use military force is nothing new, especially before combat begins. And a war bounce is also common. In mid-February 1999, before NATO airstrikes began against Serbian military targets in Kosovo, just 43 percent of Americans backed such an effort and 45 percent opposed it, according to Gallup. But by mid-April, after the attacks had begun, 61 percent favored the action. Then war fever ebbed. By mid-May, American support had fallen back to 49 percent.

A similar phenomenon occurred in France with regard to its intervention in Libya in 2011. Just 30 percent of the French population supported a Libya engagement in early March 2011, before the operation began, according to an IFOP survey. But that backing increased to 66 percent after bombs started falling. But, again, by late May enthusiasm for the intervention had dropped back to 55 percent.

In Britain, in early March 2011, only 11 percent of the public was in favor of the country’s military intervention in Libya, according to computer surveys by YouGov. Once Britain, France, and the United States launched their air attack support grew to 45 percent. But then public opinion became quite volatile, with support waxing and waning depending on developments in what grew to be a months-long campaign in Libya.

So the Obama administration and its possible allies in a Syrian intervention face an uphill challenge in convincing their publics to back such an initiative. Whatever happens at the G-20 in terms of leaders’ strategic support of U.S. military action in Syria, the history of public opinion on such actions is clear: people around the world tend to be leery of such initiatives, then once their militaries are engaged they rally to the cause. But beware: that support does not last long.