The USA has plenty of rare elements to mind as long as the military doesn’t mind paying more. The USA is already a resource powerhouse, but China has undercut it severely in refining because they hadn’t been caring much about their environment.
The USA military also really really excels at strategic logistics, I’m pretty sure they have more than just a few months stockpiled.
The military has to compete for these resources with the private sector, you might be surprised at just how much the current logistics toolchain simply buys from private companies off the shelf.
The government can, and will in any WWII-level war, choose to ration industrial metals that are in short supply. The US government in WWII effectively allocated who was allowed steel, copper, tin, cobalt, etc. to ensure that sufficient material was diverted to military needs.
Assuming a world war scenario, there likely is no competition with the private sector, which will have a choice between producing what the government wants it to produce or being put under government control and then producing what the government wants.
The USA military also really really excels at strategic logistics, I’m pretty sure they have more than just a few months stockpiled.