From the end of WW2 NATO has been a "let USA pay for Europe's defence"-club. It's a mystery why American presidents have been so cool about it until now.
I know, right? All the US ever got out of it was the ability to completely dominate the world, like the British Empire before them, only more so, and with nuclear fucking weapons.
Bearing in mind that "Europe" wasn't really a thing before the Treaty of Rome, and that the EEC, and now the EU, still doesn't have a defence role - Without NATO, the EU members would each have their own entirely separate defence forces, and would be incapable of wielding any military power whatsoever - unless they built a European army.
There is no European army, navy, or airforce; Just individual national forces.
And the US has kept these forces small and weak, by integrating them into a NATO that is dominated by American forces. No European nation could afford to match the US singlehandedly, and there is no multilateral force except NATO itself.
The US pays for the defence of Europe, and in return all she gets is to rule the world.
It's not the one-sided act of charity that Trump and his neocon backers would have you believe.
If the EU does decide to build an integrated and unified European armed force, then the US goes from having two foreign peers (China and Russia), who can challenge her domination of the world, to having three, and her global influence starts to be increasingly constrained to the Americas.
A European Army would save the US a lot of money, but could not be required, nor expected, to act in the US interest.
This narrow, small-minded and shortsighted attitude, that knows the price of everything but the value of nothing, will (perhaps) save a lot of dollars, while literally costing Americans the world.