How did the war between the United States and Iran actually start?

The short version is that it grew out of the long confrontation over Iran's nuclear programmer and its regional proxies, but the immediate trigger earlier this year was a series of Israeli and then American strikes on Iranian targets, followed by Iranian missile retaliation against Israel and US bases in the Gulf. Within two weeks millions of Iranians had fled cities, a new supreme leader had taken over vowing revenge, and the Gulf states found themselves under drone attack.

From there it spread the way these things do. Iran threatened the Strait of Hormuz, then denied wanting to block it, then mines started appearing. The US cleared mines, struck launch sites, and struck again when new mines showed up. A US pilot ejected over Iran at one point. Europe got dragged in when a French soldier was killed.

There was a month-long lull in the summer, which many hoped would become a ceasefire. It did not. This week the fighting resumed with strikes on islands in the strait and Iranian missiles fired towards Jordan.

If you want one sentence: this is a war that neither side planned to fight at this scale, and both now find it hard to stop because stopping looks like losing.