NATO was created in 1949 to defend Europe against the Soviet Union, an expansionist, evil empire that devoted time and treasure to spreading the malevolent ideology of Marxism worldwide. Fear of this was entirely rational as such a conquest would have meant descent into hell on Earth, so we rightly drew a bombs-and-bullets line in the sand. But at issue today isn’t your grandfather’s Bear.
It’s not popular today to talk about keeping the world safe for democracy, but that’s just another way of saying the world needs a dominant power, a sheriff, to set the broad terms of international business, diplomacy, currency exchange, laws of the sea, etc. Someone will take over that role if we let it lapse, and with it would go the many advantages that accrue to America as world hegemon.