What did Donald Trump do today?

He got bored waiting for information on a policy decision, so he just made it without any.

Trump has a narrow but ironclad legal authority to set tariffs, provided he is willing to claim that he is doing so in the interests of "national security." Today, he did so--or, rather, he announced that in the coming week he would be announcing that at some point after that, he would be imposing tariffs on foreign steel and aluminum. He then went on to provide some but not all of the specific details about those yet-to-be-announced tariffs--in effect announcing them in his pre-announcement announcement.

Trump's firing the first shot in a potential trade war was a surprise move, and none were caught more off guard than his own economic and trade advisors, including the Cabinet-level officials he had tasked with coming up with the plan, and who were actively steering him towards other options. But after he "ran out of patience," Trump simply declared the top-line numbers and said that more information would come later.

The news also came as a surprise to financial markets, which promptly plunged on the news. It also blindsided Congressional Republicans, who believed they had talked Trump down from the tariff ledge, and who reacted with muted horror.

Why should I care about this?

• It's bad if the president makes impetuous decisions for no apparent reason.