>>They want all their territories back.

Agreed. They're focused on gaining ground until they control places they can hold with a relatively small standing army. The Spartans could hold Thermopylae with a comparative few, but the Persians had to pass a narrow spot anchored at one side with the sea, the other side mountains (the Athenians had stopped the Persians at sea) The Russians have no place they can hold an invading army on their own territory, so they expand until they control all those places - or reach the ocean.

Here's the thing: the 'gates' they're looking to control are on the far side of now-NATO countries. If they engage with NATO directly, they'll be annihilated - and will probably feel desperate enough to go nuclear. For all our futures (Russian and Western) it's important they lose in Ukraine.