If you have to ask, we probably disagree.
It's not our job to provide "security umbrellas", and based on history since 1965, relying on us is probably a very bad idea.
A the moment, our "allies" Taiwan, Ukraine, and Israel all have the ability to drag us into wars we can't win - and an "ally" is usually a two way street - none of these "allies" has ever shed a drop of blood with us in any of the failed imperial wars we have fought since 1991.
What you call the "American security umbrella" belongs over the Rio Grande.
Ok. Remove your security guarantees from many countries. Withdraw into your comfort zone, and look on whilst the crazies topple one country after another. And then they will turn their attention to you. But you won’t have any pre deployments and countries for staging attacks on your enemies, because they may no longer exist, are bravely fighting off the crazies on their own, or the bases and training areas and local know how you through away,are now in the hands of your enemies.