Because you saw it on the news? It's self-evident from their statements and their actions. Combined with even a basic understanding of regional history. It's

Because you saw it on the news? It's self-evident from their statements and their actions. Combined with even a basic understanding of regional history. It's a fantasy to think you can push out half a million Russian soldiers [without nukes] The Vietnamese managed do exactly that its colonial occupier (the United States) some 50 years ago. France got kicked out of Algeria. The U.S. got its ass handed back to it again, Afghanistan just a few years ago. In order to reverse an occupation, one doesn't need total, crushing victory (though that would of course be more than welcome on the ground in Ukraine). You just have to convince the occupier that project will be ultimately unsustainable. And that the time has come to pick up their toys and go home.