Iraq? A vipers’ nest of chemical and biological weapons.

The Soviet Union? An unshakable, indomitable behemoth.

Vietnam? The first commie domino to fall in China’s takeover of Southeast Asia.

Wrong, wrong, wrong.

And now, there is a gathering of official wisdom to support a U.S.-led military strike on the regime of Syrian President Bashar al-Assad. The Obama administration asserts that Assad’s government deployed chemical agents in an attack outside Damascus that killed a large number of civilians and crossed moral boundaries.

The White House’s argument seems convincing. But there are always unintended consequences. Unknowns.

In the conservative Weekly Standard, there are no hand-wringing Hamlets. In an open letter to the presidenton the magazine’s Web site, 66 signatories urge Obama to unleash military power to help the rebel side in the long-running civil war, to prevent further atrocities and “ensure that Assad’s chemical weapons no longer threaten America.”

Many of the signers had also agitated for the 2003 invasion of Iraq and the destruction of Saddam Hussein’s supposed biological and chemical weapons.

Is it right to afford policy experts and analysts who were wrong on Iraq any credibility on Syria? Perhaps.

“Is anybody ever right in Washington, and how would you know?” Clifford May, president of the Foundation for Defense of Democracies, said.

He promoted the Iraq invasion, signed the open letter to Obama and has, for at least two years, urged more muscular U.S. action against Assad.

“To say that anyone who supported the Iraq war should shut up on Syria — that is vastly oversimplified as an intellectual exercise,” May said.

Still, the lessons of Baghdad hang over the debate like a mushroom cloud.

In Britain on Thursday, Parliament voted against military intervention in Syria. Edward Miliband, the opposition leader, citing former prime minister Tony Blair’s support for the U.S.-led invasion of Iraq, said he is “unwilling to have those mistakes made again.”

In Washington, the roster of high-level believers in Iraqi WMDs is long indeed. But the White House case on Syria is convincing to many, especially in the face of victim testimony and medical evidence suggesting that the weapon was nerve gas.

“The red line is the president’s. I can’t know more than he does,” said Danielle Pletka of the conservative American Enterprise Institute, an Iraq war supporter and another signer of the letter on Syria. “And in the case of Iraq, I certainly could not know more than the 15 members of the [U.N.] Security Council.”

Perhaps that has something to do with the lifeblood of the town: politics.

People sign onto presidential and other campaigns convinced that their man or woman will win. When that doesn’t happen, the losers aren’t shamed in the least. They merely reconstitute themselves into experts, advisers (to other campaigns) and acolytes of the Loyal Opposition.

There’s another phenomenon that allows Washington policymakers to always be right. It’s political, too: You simply ignore the other side of the argument.

“You can’t be wrong if you live in an ideologically self-enclosed box,” said Sidney Blumenthal, a former adviser to President Bill Clinton. “If that’s the only place you live, if you are always confirming your own dogma, you are always right.”

In 2002, Kenneth Pollack, a former CIA analyst and Middle East expert at the Brookings Institution, published an influential book, “The Threatening Storm,” which argued for invading Iraq. He got it wrong.

The lesson: “Predictive analysis is extremely difficult,” Pollack said. “We do have to recognize that the human capacity to determine the probability of future events is limited. History has this very bad habit of throwing what seem to be million-to-one probabilities at us.”

So Pollack is reserving judgment on who did what in Syria.

“I have no fricking clue,” he said. “I have no independent means of assessing whether the Syrian regime used chemical weapons.”

On whether the Western powers should intervene, he comes down firmly . . . on the wishy-washy side.

“There is a strong argument for intervention in Syria and a strong argument against,” he writes in his latest paper on the matter. “Both would require far more decisive action by the United States.”

Robert Baer, a writer and former CIA operative, has long urged U.S. military intervention in Syria because of the danger of a large-scale chemical-weapons attack, citing sarin as the likely chemical agent.

He predicted on CNN more than a year ago that if the conflict were prolonged, Assad would gas his own people.

“Of course, even a stopped clock is right twice a day,” Baer offered. “And I’ve been wrong a lot on the Middle East. It comes with the territory. You only want to remember when you were right.”