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Should and could are different things. It is illustrated pretty clearly right now with the war in Ukraine. Europe clearly lacks abilities to produce enough and right kinds of weapons for Ukraine and is instead paying for the American weapons that are being shipped over there...

I don't think Ukraine is a good example - Europe lacks the will to actually go in there. Producing the equipment is only a problem because of the demilitarisation encouraged by the US. It shouldn't happen again.

Why is Europe paying for US arms to Ukraine instead of producing their own weapons for Ukraine, unless buying American is cheaper?

because for so long, Europe hasn't produced much. Now, they are also trying to do trade deals with the US on top of that. They should ramp up local production and no longer use the US. Screw trade deals

Ramping up local production requires large investment...

Yes. Large local investment. This is what is needed. :)

It will stimulate local economies...