PARIS — Upon his return from Vietnam, after serving there as a lieutenant, John Kerry delivered impassioned anti-war speeches to U.S. senators and excited crowds at peace rallies. That was 32 years ago. Today, John Kerry is President Obama's Secretary of State, and the most vocal advocate of a military intervention in Syria to punish the regime's use of chemical weapons.

But despite all of the eloquence and conviction that Kerry is capable of displaying, one central fact about the Syrian crisis cannot be concealed: American public opinion — in line this time with European sentiment — is largely hostile to the prospect of their country taking military action against Bashar al-Assad’s regime. This turnaround of opinions, in comparison with previous allied interventions, is a key part of the Syrian equation and complicates matters for the few heads of states who are in favor of using military force.

The numbers are stunning. The research institute on transatlantic relations German Marshall Fund found that, in 11 surveyed countries (the U.S. and 10 European countries) there has been an across-the-board increase in the opposition to military intervention in Syria compared to 2012. The percentage of those opposed rose from 63% to 75% in Germany, 59% to 70% in Britain, 50% to 65% in France, 55% to 62% in the U.S. In Turkey, where the Prime Minister Recep Tayyip Erdogan even called for an overthrow of the Syrian regime — which the American and EU authorities have carefully avoided doing — 72% of public opinion is against an intervention.

In France, as a survey published on Sept. 7 by Le Figaro showed, the opposition to a potential intervention has accelerated very rapidly these last few days: In August, 55% of the French population was in favor of international military intervention; now, only 36%.

What happened

This evolution of opinion is the exact opposite of what happened at the time of the interventions in Kosovo, Serbia, and at the beginning of the war in Afghanistan — when the coalitions’ action was clearly understood and publicly supported. The intervention in Bosnia took place in a context of “opinion diplomacy.” More recently, the French military engagements in Libya and Mali were backed by more than two-thirds of those surveyed in France.

What happened? There is, of course, a feeling of weariness in Western countries since 2001, as forces engaged in conflicts have fought with no clear vision of objectives, and without clarity of success. There is the nature of the Syrian rebels, their divisions, the presence of Jihadists among them, their methods and the video footage of their blunders. There is the dismay facing the evolution of what started in 2011 as an “Arab Spring.”

There is also, and most of all, the Iraqi fiasco, of which the political consequences, 10 years later, are exploding in the face of the American administration. David Cameron paid a high price in the House of Commons on Aug. 29 — François Hollande, too, even though France stayed out of the Iraqi adventure.

If they want to succeed, the political leaders in favor of military strikes will have to work twice as hard to convince not only other government representatives, but also their fellow citizens.