NATO has nothing to do with "policemen".
NATO was founded in 1948 to defend Europe against Old Soviet aggression.
After the Old Soviet Empire collapsed in 1991 people could make a reasonable argument that NATO had accomplished its purpose and could now disband and enjoy retirement sitting on its laurels.
Except that, for some eerily strange reason, every d*mn one of the Old Soviet Empire countries came clamoring for admission to the now presumably obsolete NATO -- why was that?
Well... well... well... because, come to find out, that Old Soviet Empire was not quite as dead or defunct as first reported.
Turns out that Old Uncle Joe Stalin's premier lovechild, born just a week before Old Uncle Joe's death in 1952, later became a young KGB LtCol named Vladimir Vladimirovich Putin.
Today Vlad the Invader rules over Russia and has restored Old Uncle Joe to positions of honor and Stalin's expansionist policies as driving forces in Russia's behavior on the world stage.
So today NATO's original purposes are reborn with a vengeance and of course the USA is part of NATO, at least nominally its leader, and so necessarily a primary contributor to any collective defenses NATO undertakes.
None of this has anything to do with being the "world's policeman" and has everything to do with fulfilling NATO's 75 year long committment to defending Europe against the Kremlin's crazies and Muscovy's maniacs, "mother Russia's" radical Ras-Putins.
Nothing good can result from the US abandoning our allies.
