The Kate Steinle case was always about more than just the Kate Steinle case.

The 32-year-old was shot in the back while walking with her father along San Francisco’s Pier 14. She collapsed and died in her father’s arms. It was July 1, 2015.

The bullet came from a gun stolen four days earlier from a federal ranger’s official vehicle, where it had been left unsecured and loaded. But the story wasn’t about carelessness.

The gun was fired by a man who was living on the street, but the story wasn’t about homelessness. And it wasn’t about gun violence.

The story was about illegal immigration and sanctuary cities, because of what had happened two weeks earlier.

Donald J. Trump announced on June 16, 2015, that he was running for president.

Illegal immigration was not a central part of Trump’s announcement speech in New York. His brief remarks about Mexico “not sending their best” people, and characterizing some of the immigrants as criminals and rapists, received intense news coverage, but most of the speech was about trade.

Then Kate Steinle was shot to death.

The revelation that the suspect arrested for firing the bullet that killed Steinle was a Mexican citizen who had been deported five times already, and who would have been deported a sixth time if not for San Francisco’s sanctuary policy protecting him from being turned over to Immigration and Customs Enforcement upon release from a local jail, changed American politics.

Trump seized on the story and pounded the message that the nation needed a wall on its southern border. On Aug. 31 he gave a major policy address in Phoenix to outline what he called “one of the greatest challenges facing our country today, illegal immigration.”

Trump told the crowd, “Today, on a very complicated and very difficult subject, you will get the truth. The fundamental problem with the immigration system in our country is that it serves the needs of wealthy donors, political activists and powerful, powerful politicians. Let me tell you who it does not serve. It does not serve you, the American people.”

As Trump rose in the polls, no other candidate in the race co-opted the issue and ran with it, which, if it doesn’t confirm Trump’s assessment of “the fundamental problem,” certainly doesn’t contradict it.

Now he’s president of the United States, and that’s probably why.

On Thursday, the man arrested for firing the bullet that killed Steinle was acquitted on charges of murder, manslaughter and assault with a firearm. The jury apparently believed the public defender’s argument that the gun fired accidentally and the bullet ricocheted off the concrete ground.

But the acquittal is a footnote to the story of a bullet that changed presidential history.

The Trump White House — there’s a Trump White House — has established a new Homeland Security office called Victims of Immigration Crime Engagement (VOICE) to give assistance to victims of crimes committed by people who are in the country illegally, and the Justice Department is trying to crack down on “sanctuary” cities by threatening to withhold grants.

San Francisco slightly modified its sanctuary policy, but still today it would result in the release of the man who fired the gun that killed Kate Steinle. And now a new law, sponsored by U.S. Senate candidate Kevin de León and signed by Gov. Jerry Brown, has made all of California into a sanctuary state.

California politicians may not like Donald Trump, but they did more than they know to elect him.