Historically being WW2. I'm not sure they've contributed much to the "war effort" since then.
A legacy of WW2 was the explosion of the whole defence industry. While a lot of the civilian factories got repurposed in WW2 to build trucks and planes and ships, the tech in all that stuff was very basic. For example the aircraft carrier Yorktown was "fixed" (at least good enough for the Midway battle) in 3 days. I somehow doubt a modern carrier could do that, simply because of the tech required. The US built 150 more carriers in the next 3 years.
The defense industry today gets a trillion $ a year. There's no civilian ship-building to speak of, and military vehicles are now highly specialized.
So why keep Ford et al afloat? Politically it's sold as "national security." In reality it's like more "they didn't die on my watch." And of course, having the ability to make local affects the supply chain. [1]
For example if China invades Taiwan, the US loses pretty much all electronics- especially PCs and Phones. That's a lot of leverage for a foreign country to have.
[1] US cars aren't really local - that's a fiction exposed by the proposed tarif on "car parts", which the industry squashed. In reality car parts are made all over the place (including the US) , and then assembled in the US (or anywhere else.)
“too-big-to-fail eventually becomes too-big-to keep alive"