Of course there is no evidence that there was any imminent attack, even according to Mark Esper, the U.S. Defense Secretary. That's because Trump's claim of "self-defense" was torn right out the well-worn US Playbook of War Justifications. It's the chapter right after the two chapters dealing with "The Gulf of Tonkin" and "Weapons of Mass Destruction."
When you have a hammer, everything looks like a nail. The U.S has such a enormous arsenal of war machines that it makes military actions an almost irresistible temptation. It feels so good to do something--almost anything--rather than doing nothing. The focussing illusion makes launching an attack feel like the right thing to do even when the big picture shows that a killing of a prominent leader of a country could stir up massive intense hatred toward the United States. Trump's inner circle of warmongers (which apparently might not include Esper) are true believers that a show of military force is powerful, effective and bitter medicine for Iran. Maybe so, but those who stir up intense hatred should be careful when their country has thousands of soft targets (schools, shopping centers, airport lobbies, banks, stadiums) that are realistically indefensible. And this is especially true in that we also have dozens of realistically indefensible big cities that could be horrifically paralyzed by dirty bombs, biological weapons and cyber attacks well within the means of a technologically sophisticated country like Iran.
I'm sure that Trump felt really happy--probably asmic-- when he killed Qassim Suleimani. But based on how he has handled many things, including North Korea, I am fairly certain that he didn't think through the long term consequences of that feel-good moment.