I would agree with you on trade that a trade war would be really stupid as USA would ultimately lose.

But I don't think Trump's threats of tariffs is just about trade but rather politics.

Richard Wolff over at Democracy at work in a rather long video suggests that this tariff threat is politics as usual.

Trump is pitting the steel and aluminium corporations against the corporations that buy & use steel and aluminum to see which industries will give him the most campaign contributions. Trump is basically extorting industry for votes.