The optimistic alternative is EU countries scale up their military production and cover the gap. We were already seeing a ramp up, but it'll have to accelerate.
Downside for the US is later down the line, exports will go down as the EU will have more domestic manufacturing.
Honestly, if you're in the US, best you can do is focus on your local community. Help out your neighbours, find like-minded people, stay active on the local level. There is almost 0 possibility for a normal person to make a difference on the national level, but you absolutely can on the local level.