Here's NATO's tank strength, largely confined to...the US, Turkey and Greece.

And Poland.

Again, geography. There are really only two overland paths from Russia into Europe that you can move mechanized divisions through. The first is the Great European Plain, which was discussed above - the large flat plateau that runs from France to Russia. It only goes through Poland. Everything south of that is blocked by the Carpathian and Balkan mountains. The only other overland path is through Georgia along the southern shore of the Black sea, skirt around the south of the Rodope mountains and along the north edge of the Aegean sea and up through Hungary. Which is through Turkey and Greece.

Which is why the tank divisions are in Poland, Turkey, and Greece. The three land bridges to Russian territory. Turkey and Greece have oversized tank brigades to protect each against the other as well.

Huh? You're aware that the Russians have traditionally had a large submarine force, right?

Oh, sure. And they do have a navy, of course. But they don't have much of a blue water navy at all. Their fleet is almost entirely small ships, really suited for waterbodies like the Black Sea and smaller. Subs are important counter-naval assets, but they're not going to provide a huge amount of direct support for a European invasion.