What did Donald Trump do today?

He gave up listening to people trying to reason with him and ordered the Navy to downgrade its combat effectiveness.

Trump signed an order today requiring the Navy to scrap the modern electromagnetic catapults used on aircraft carrier flight decks and replace them with steam-powered catapults, the 1950s-era technology that preceded them. Returning to "goddamn steam" has been a fixation of his for almost a decade, and he's tried repeatedly without success to convince anyone who actually works in the Navy, or in engineering, to his point of view.

Today's order simply short-circuits the whole process and demands billions of dollars in changes to the next generation of Navy ships. 

Trump's comments about, for example, how he thinks magnets don't work underwater have been played for laughs in the past, but today's order has real consequences. Over and above the cost of redesigns and retrofits, and the years-long absence from the fleet that would require for each ship, the change would instantly reduce combat effectiveness. No matter how devoutly Trump believes otherwise, electric catapults are faster and more powerful, and can launch jets that steam catapults never could. They're also less likely to damage aircraft, require less maintenance, and are available for use instantly, whereas steam catapults require time to come up to operating pressure.

As a practical matter, Trump's order will never be followed—almost certainly, not even in a token fashion. What will likely happen is that the Navy will stall any work related to the affected systems and run out the clock of Trump's term of office. But that is likely to delay the arrival of new carriers altogether and increase the cost even further. Worse, it puts the Navy in the awkward position of having to pay lip service to orders—technically, lawful ones—that it knows will countermanded the moment anyone other than Trump is President.

What makes the situation particularly bizarre is that Trump is not subscribing to a minority viewpoint in naval engineering circles. He is, quite possibly, the only person at any level of government who believes this, and he appears to have come up with the idea all by himself.

China, the major threat to US naval dominance and the only world power building carriers that can compete with American ones, has not decided to downgrade its launch systems.

Why does this matter?

• A sane, cognitively fit commander-in-chief wouldn't ignore an entire service branch telling him he's wrong and doesn't know what he's talking about. 

• If a foreign power degraded American military capabilities this much in a single stroke, it would be the greatest act of sabotage ever.