If anyone has seen Castle Rock on Hulu, there are some striking similarities in that there is a weak place between universes, and certain people are key to restoring a natural balance, if you will.
The concept that amazed me was Himmler's idea that he could send troops, tanks, and artillery (after clearing and widening the tunnel, and maybe removing some mountain) through this gate to what exactly on the other side? Not as if the way is open and paved for them. Even if they can only send troops (with deceased counterparts), image our 1960s with Nazi soldiers stumbling out the Poconos. I don't foresee their invasion going as planned.