Trump’s tariff decision was poor and his defense of it was even worse, but perhaps worst of all was the way that he came to make that decision:

But the public show of confidence belies the fact that Trump’s policy maneuver, which may ultimately harm U.S. companies and American consumers, was announced without any internal review by government lawyers or his own staff, according to a review of an internal White House document.

According to two officials, Trump’s decision to launch a potential trade war was born out of anger at other simmering issues and the result of a broken internal process that has failed to deliver him consensus views that represent the best advice of his team.

Whatever one thinks about the merits of raising tariffs on certain imports, this is an unacceptably slipshod and arbitrary way for any president to make important policy decisions. Because there was no internal review, no one can pretend that the administration thought through the implications of this decision and weighed the potential costs and benefits of doing this. The president made a significant, potentially costly decision more or less on a whim, and because he has dishonestly invoked national security to justify what he’s done he can probably get away with his capricious decree. Trump’s own public defense of his decision shows that he didn’t understand the likely consequences of what he was doing and doesn’t care about those consequences even when they are repeatedly explained to him. Even if you happen to agree with the action taken, the haphazard, incompetent manner in which it was done should alarm you. There is a strong whiff of “do-somethingism” about Trump’s decision, and that almost always means that the decision is a bad one made for the wrong reasons.

Jeff Spross sums up very well why Trump’s decision is so poor:

Instead, we’re getting a hastily constructed policy premised on a transparently dishonest idea.

Just as he did with the travel ban, Trump is trying to use national security as a cover for things that have nothing to do with it. Perhaps he does this because he thinks he can use national security rhetoric as a way to bludgeon critics of his actions, or perhaps he is just looking for a convenient pretext, but whatever the reason for saying this it is dishonest and wrong.