they don't really have to. there's a lot of Russia. like a really really big health bar. traditionally going back to the czarist regime the Russian army buries you under their conscripts until you run out of arrows, sharp things, or bullets. then sends more conscripts.

the internal regime is really unstable and Ukraine is hitting things Russians care about, so it's not a foregone conclusion, but it is Ukraine's early game hard mode dark souls boss to beat.