The Eurozone (NATO) is protected from Russia and other aggressors by US military spending, and trades along sea routes which are kept open by US military spending, to list only two ways in which the EU benefits massively from our tax dollars here in the US.
If the EU decided to allocate a proportional share to their common defense, maybe the US would have less reason to be intervening overseas and could spend some of our money here instead.
The EU spends about 4 times that amount that Russia (the only direct military threat) spends. Russia has much smaller population and a much smaller economy and pretty much depends on selling oil and gas to the EU to survive.
If the EU decided to allocate a proportional share to their common defense, maybe the US would have less reason to be intervening overseas and could spend some of our money here instead.