The Ottoman Empire’s slaughter of 1.5 million Armenians early last century is one of the most well-documented attempts at genocide on record, down to the U.S. ambassador’s contemporaneous description of a “campaign of race extermination.”

But it’s taken almost 100 years just to get one half of the U.S. Congress to acknowledge and affirm that such an atrocity took place and that, as stated by the nonbinding resolution, the U.S. government should no longer associate itself “with denial of the Armenian Genocide or any other genocide.”

The resolution, which passed the House on a 405-11 vote Tuesday, is the culmination of decades of intense lobbying, worth tens of millions of dollars, if not more, from the Turkish government that could serve as its own PhD-level course in how Congress works — or, in this case, for the past few decades, how something doesn’t work and usually ends in deadlock.

And, like so much of the past few years, what changed the dynamics was President Trump, beginning with one phone call earlier this month. Without consulting Congress, Trump called President Recep Tayyip Erdogan of Turkey and agreed to withdraw almost all U.S. troops in northern Syria and essentially allowed Turkish forces to invade Kurdish strongholds to attack their longtime rivals, who for years had been the chief U.S. allies in the fight against the Islamic State.

With that, Turkey lost its already wavering support on Capitol Hill, and the dam burst with a flow of proposed sanctions, which also were approved by the House on Tuesday, and new momentum behind the long-stalled effort to officially recognize the Armenian genocide.

House Speaker Nancy Pelosi (D-Calif.) smiled, pausing for five seconds and shrugging her shoulders, when asked if Trump’s move had eased the way toward recognizing the genocide.

“There was just an aura of what the Turks could be doing, is engaged again in genocide, by the greenlight that the president gave them. So that was sort of a callback,” Pelosi said Tuesday.

The resolution still faces an uphill climb toward winning passage in the Senate, but just this mere victory represented a seismic shift against the usual powers of Washington insiders to thwart things that seem so obvious to most people.

The resolution is one of the rare things that Reps. Adam B. Schiff (D-Calif.) and Devin Nunes (R-Calif.), bitter rivals as chairman and ranking minority-party member of the House Intelligence Committee, completely agree on.

After the vote, Schiff described the explanations he heard the past two decades from members: “I don’t want to offend Turkey, now is just not the right time. . . . It was never the right time.”

That changed, in overwhelming fashion the past few weeks, when Pelosi agreed to bypass the committee process and bring Schiff’s resolution to the full House.

“I think that members felt that they were no longer comfortable carrying Turkish water on the genocide when Turkey was once again engaged in a campaign of ethnic cleansing,” he said.

“Armenian Genocide recognition could finally receive vote on House floor,” Nunes tweeted Monday, linking to a story about the pending vote.

Schiff was a central player the last time the House came close to voting on the Armenian genocide resolution, in October 2000 when he was a state senator running in what was then the most expensive congressional race of all time for his Los Angeles-area district.

The House speaker at the time, J. Dennis Hastert (R-Ill.), campaigned for the incumbent, James Rogan (R), and promised to hold a vote on the resolution just weeks before the election, giving all credit to Rogan. The House minority leader at the time, Richard A. Gephardt (D-Mo.), cried foul and said Hastert was playing domestic politics with foreign policy — but he also pledged to pass the resolution if Democrats won the majority and he became speaker.

However, in the last weeks before the November 2000 election, the State Department intervened and persuaded Hastert to stand down because Turkey served as such a critical NATO ally, particularly with its inroads to Middle Eastern nations.

Schiff went on to win the race and rise to national prominence as the lead investigator in the impeachment inquiry of Trump, but he could never get the full House to vote on the Armenian resolution — not even when Pelosi served as speaker for four years last decade.

Instead, Turkey’s government, as well as its business alliances, built a lobbying behemoth that thwarted any effort at acknowledging what its predecessors had done.

Gephardt and Hastert, once out of Congress, switched sides and went to work for Ankara’s government.

From 2008 through 2015, Gephardt’s firm had collected $8 million, and just a couple months after his cooling-off period ended in early 2009, Hastert signed a $35,000-a-month retainer to lobby for Turkey and, among other issues, block the Armenian resolution vote.

Both Republican and Democratic secretaries of state, including Condoleezza Rice and Hillary Clinton, fought the effort to officially condemn the Ottoman Empire for its slaughter.

In 2017 and 2018, Turkey and its government-affiliated entities spent about $13.4 million lobbying Washington, according to Open Secrets, with its biggest firm, Ballard Partners, led by Trump’s most well-connected supporter from Florida. They also added the Daschle Group, founded by former Senate majority leader Thomas A. Daschle (D-S.D.), to bolster their bipartisan credentials.

Ankara’s lobbyists in Washington pushed hard the past few weeks, after Trump called Erdogan and the Turks attacked the Kurds. Just 11 Republicans listened to that campaign, most of whom are the administration’s staunchest allies.

“I have a lot of confidence in the president and the administration knowing what to do in Turkey, and I didn’t want to interfere,” said freshman Rep. Greg Pence (R-Ind.), the brother of Vice President Pence.

The State Department put in a cursory effort this time around, nothing like previous full-force efforts to stop the Armenian resolution.

“No one’s called me, no one’s called me. I think maybe they knew that I wouldn’t be influenced by it,” House Foreign Affairs Committee Chairman Eliot L. Engel (D-N.Y.) said before the vote.

Schiff said today’s events made that fight pointless.

“There are human rights abuses going on today, and the echo through time, I think, was just too powerful to ignore,” he said.