If without external help, Russia should have an easy time fighting Ukraine, but Ukraine has the support of the U.S. and European countries, and the battlefield situation is already reversing. Overall, Ukraine is currently changing from defensive to offensive, and Russia is changing from offensive to defensive. The latest news is that the U.S. 101st Airborne Division has been deployed to Romania and Poland is increasing its troops to the border, making the battlefield situation increasingly unfavorable for Russia. As long as the war does not reach Russia, Russia should not use nuclear weapons, once the war burns to Russia, I think Putin once said a sentence will know what will happen, Putin once said "Russia is gone, what do we need the world". You can imagine that once pushed, Russia is out of everything. Therefore, Russia will not admit defeat, and once it is really invaded by the West, it will be a nuclear war, which is not what the United States and the West want to see, most likely from the internal disintegration of Russia. But this is only speculation, because after all, there are still Iran, North Korea and SCO, these will not stay out of it.
