Trump, in accordance with his campaign promises to the rust belt and flyover country, has just slapped a tariff on steel and aluminum.
If you look at the Nucor product catalog, you can see that the USA has ceded high end steel production to foreigners.
Ceding high end steel production to foreigners is militarily unwise.
Ceding the high end is also likely to have externalities. A network of skills unravels. If company A does something high tech, it cultivates employees, customers, and suppliers that make it substantially easier and cheaper for company B to do something high tech, and this benefit is not captured by company A, unless, as in South Korea during the dictatorship, the state gives company A substantial monopolistic privileges, something difficult to do in a democracy, particularly a democracy where covetousness is deemed the highest virtue and high status.
And if company A stops doing something high tech causing other companies to stop doing high tech stuff you have the rust belt, which is the network of high skilled white males unravelling. You have smart white men deskilling, taking opiates, and committing suicide.
That the rust belt is rusting means that white males commit suicide or move to the big coastal megacities. Which means they move from where their votes are useful to Trump and Republicans, to where their votes are useless, because massively outvoted by hordes of aliens imported to live on crime, welfare, and voting Democrat.
Stopping the rust gives republicans a little more time, regardless of whether it is economically justified or not. Even if it was a total money loser (and quite likely externalities make it economically lucrative) it would still be politically a big winner, by halting the great centralization.
Recollect that the government was importing hordes of black male military age Mohammedans screaming for infidel blood and white pussy, and bombing marginal electorates in flyover country with them. The permanent government continued doing this for the first year of the Trump presidency, but in 2017 December, Trump finally managed to put a stop to it. This also gives Trump and Republicans a little more time.
A policy of economic autarky ruined Nazi Germany. The very similar Smoot Hawley Tariff Act of 1930 was also an economic disaster, ruining the USA. And, similarly, India’s program of economic autarky kept India stagnant and desperately poor for decades. But these three examples of bad, indeed utterly disastrous, protectionism were accompanied by massive regulation. Trump is deregulating. That is a big and important difference.