I only see a number of scenarios as to why he would actually carry out an act of war against Iran. Some may overlap, and there are likely other angles to consider that I haven't thought of here.
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He stopped Iran from building an actual nuclear weapon (unlikely, as this has been the deep state narrative for perpetual war since the 90's, if not longer.)
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He neutered Israel's reason to continue attacking Iran.
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He stopped Iran from getting a nuclear weapon (as in, getting one dropped on them by Israel.)
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Trump is baiting out Israeli operators for their false flag attack on America.
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Trump's announcement of this strike is either a head-fake or a signal.
Either way, recall what happened after Trump killed an Iranian general his last term. No WW3 broke out, even though everyone memed the hell out of it for weeks.