They are three distinguished senators, but their staffers call them the “three amigos” — because they travel abroad together a lot, because they often speak up together and because they not infrequently find themselves standing alone. This month, John McCain, Joe Lieberman and Lindsey Graham are talking about Syria. They are making the case for why the United States should lead an intervention to stop the slaughter being perpetrated by dictator Bashar al-Assad.

“The Syrian people are outmatched. They are outgunned. They are confronting a regime whose disregard for human dignity and capacity for sheer savagery is limitless,” said a statement the three issued March 6. “Still they carry on their fight. And they do so on behalf of many of the same universal values we share, and many of the same interests as well . . . . Shame on us if we fail to help them now in their moment of greatest need.”

Not many want to listen to them, even among McCain and Graham’s fellow Republicans. (Lieberman is a independent who caucuses with the Democrats.) Fewer still want to join them. President Obama has brushed them off, hinting that they are “beating the drums of war” with scant regard for the consequences. The blogosphere feasts on them, calling them reckless warmongers and much worse.

“We are taking a few arrows,” McCain concedes. But then, the three amigos have lived through this before.

A year ago, after visiting Libya, they were among the first to call for NATO to set up a no-fly zone to stop Moammar Gaddafi’s offensive against his own citizens. They were called reckless then, too. Defense Secretary Robert Gates condemned what he called “loose talk” of bombing and said they were inviting “a big operation in a big country.” Obama resisted them, until France and Britain began pushing for an intervention — and Gaddafi’s forces prepared to assault Benghazi, the country’s second-largest city. NATO finally acted, and seven months later, Gaddafi and his regime were gone.

In January 2007, McCain, Lieberman and Graham were among a scant few in Congress to support George W. Bush’s surge of troops in Iraq, which McCain had been advocating for years. In a white-hot Senate hearing, the plan was assailed by Republicans and Democrats alike — including three senators at the time, Barack Obama, Joe Biden and Hillary Rodham Clinton. Today the consensus in Washington is that the surge rescued the United States from catastrophe in Iraq and made possible the withdrawal that Obama completed as president last year.

For Lieberman and McCain, this history goes back to the siege of Sarajevo during the Bosnian war of the mid-1990s. The two senators joined Republican leader Bob Dole in pushing for U.S. intervention, against a resistant Pentagon and a reluctant President Bill Clinton. The Pentagon famously claimed that up to 400,000 troops would be needed to impose peace. When the United States and NATO finally bombed Serbian forces in the summer of 1995, the bloodshed ended in two weeks.

“I feel like we are reliving history,” Lieberman said in a meeting I had with the three senators in his office last week. “What it shows is that civil wars we get involved in can be settled more successfully than civil wars where we don’t get involved.”

“We have a record of being right,” McCain said. “Go back to those previous debates. It’s the same arguments they used in Libya, the same against doing something in Bosnia and Kosovo, the same that was said about Rwanda.” The episode everyone in Washington now regrets is the last one — a genocide that the United States failed to prevent.

In some ways, the case for acting in Syria ought to be easier. It is not just a matter of preventing a humanitarian catastrophe, after all: The defeat of Assad would be “the biggest strategic setback for Iran in 20 years,” Central Command chief Gen. James N. Mattis told McCain in a Senate Armed Services committee hearing two weeks ago.

So far, however, facts and history haven’t helped much in the Syria debate. Instead, all sides are playing their usual roles. The Pentagon is talking about Syria’s allegedly formidable air defenses. Self-styled “realists” are claiming that helping the Syrian opposition will only inflame an incipient civil war. Obama is saying that “the best thing we can do” is to “unify” the “international community.” At times it seems nothing has changed since the Bosnia debate 17 years ago.

Only something has. The United States has fought two grueling wars in the past decade, and the country is weary of them. The economy is weak; the foreign policy elite’s favorite topic of discussion has changed from “the world’s only superpower” to “managing decline.” Syria may prove to be the case where the three amigos finally fail to win the argument. The massacres will go on, and the United States will decline to respond.

“I understand the public’s reflexive reaction of ‘not again,’ ” Lieberman said. “And we don’t have to do it. But if we don’t, we are going to find out eventually not only that doing nothing was wrong but that we missed a strategic opportunity.”