This isn't a preemptive war; this is a "preventive" war. And there are no, none, nada, legal justification for a "preventive war."
My wife said this
Preemptive War is legal in international terms, BUT it has strict guidance on how this works, and 3 condition must be met before you can call a war a preemptive war
1.) An attack on your own country is imminent and will be destructive.
2.) There are no alternatives
3.) had already tried a diplomatic solution, but it didn't work.
Unless the US has intelligence that Iran is about to attack the US (Not just bases in the Middle East, but the US Mainland) and it is imminent, it's not a month after or a year after, 1 did not fulfill.
You can argue whether or not 2 is fulfilled, and 3 is fulfilled.
A Preventive War is to prevent something from happening, that's not legal in any shape, way or form, and it is what the current US administration is using to fight Iran, because it prevents Iran from acquiring nuclear weapons, well, at least this is what the President said, and it's not legal in any international law.