There’s a complex set of reasons but my take is:

Trump needs a national emergency so he can implement restrictions on voting in November. He’s already cited election interference in 2020 and 2024 as the reason for the invasion. He will use this to issue executive orders banning mail in voting and to ensure ICE are present at polling booths.

Additionally, Trump is very easy to manipulate. If a couple of other leaders like Bibi and Putin said “hey everyone would think you were so amazing if you flattened Iran right now”, he would do that, 100%. Some people are saying that someone is blackmailing Trump because they have the unredacted Epstein files, but I don’t think you really need that to manipulate Trump.