If the EU countries all spent 3%+ GDP on defense, they would have kicked the US out decades ago. They purposely spend a lot less, 1-1.5%, so they can funnel the rest into welfare spending, and use the US taxpayer to make up the difference. It's a very one-sided arrangement, but the US goes along with it because it's basically a very expensive "pay for influence" scheme that no president has been willing to end. Trump tried to reduce it somewhat.