A volley of U.S. cruise missiles had barely been launched into Syria before the Internet filled up with fact-free theories about the real reason for an international crisis.

A popular one on the right-most fringes: The U.S. government actually carried out the chemical weapons massacre in Syria last week — a “false flag” to trick President Trump into retaliating, thus entangling himself in a foreign war.

A slightly more convoluted strain on the left: Russian President Vladimir Putin ordered the chemical weapons massacre to help Trump — distracting Americans from an investigation into Trump's campaign ties to Russia by provoking the missile strike.

That theory — evidence-free — was laid out on a small anti-Trump website shortly after the missile strike.

But it went mainstream Friday night, when Lawrence O'Donnell advanced similar speculation on his MSNBC show, “The Last Word.”

“Wouldn't it be nice,” O'Donnell asked a nodding, smiling Rachel Maddow, “if it was just completely, totally, absolutely impossible to suspect that Vladimir Putin orchestrated what happened in Syria this week — so that his friend in the White House could have a big night with missiles and all the praises he's picked up over the past 24 hours?”

The theory was impossible to rule out, O'Donnell said, because of the Trump campaign's ties to the Russian government.

A few minutes later, the host elaborated on his theory under banner text: “Wag The Dog?” — recalling a similar conspiracy theory that President Bill Clinton launched missiles in 1998 to distract from his own scandal.

“It changes the conventional wisdom about the dynamic between President Trump and Vladimir Putin,” O'Donnell said. “President Trump has finally dared to do something Vladimir Putin doesn't like. It changes everything.”

O'Donnell didn't offer any evidence on his theory, promising only that “you won't hear ... proof that the scenario I've just outlined is impossible.”

What O'Donnell did hear, if he followed the reaction to his show, was derision from across the political spectrum.

The clip spread quickly across right-leaning sites where a writer on Twitchy asked: “Honest question: MSNBC is OK with this crap?

The network declined to answer The Washington Post's questions about the segment, which Adam H. Johnson — a writer for the left-leaning magazine the Nation — called “a great weasel way to float bat s--- conspiracy theories.”

CNN reported Saturday morning that the United States is investigating whether Russia, which has long supported Syrian President Bashar al-Assad in the atrocity-filled civil war, was complicit in the chemical attack in Idlib province last week. A Kremlin spokesman denied it to the network, as has Assad's regime.

The attack last week caused civilians and children to choke, foam at the mouth and die in a cloud of poison, possibly sarin.

Trump lamented seeing photos “of innocent children, innocent babies” killed in the strike, which he said made him reconsider his longtime stance against intervening in Syria's civil war.

Early Friday, Trump ordered dozens of cruise missiles launched at a Syrian military airfield, marking the United States' first direct intervention in the six-year-old conflict.

If, as O'Donnell theorized on MSNBC, Putin had planned all this to boost Trump's poll numbers, the Russian president didn't show it.

Russia instead retaliated by pulling out of an agreement designed to prevent skirmishes with the United States in the airspace over Syria.

What prompted the U.S. to act now, six years after the start of the civil war? The Washington Post's Amanda Erickson explains President Trump's decision to strike Syria. (Amanda Erickson,Jason Aldag/The Washington Post)

Read more:

Americans hated the idea of strikes against Syria in 2013. But Trump’s could be different.

Trump’s strike on Syria disrupts the narrative that he is Putin’s pal