Why does the United States dare not intervene in Ukraine and sit back and watch Russia control the situation?

I'm afraid you have only seen the surface. Now, Ukraine is completely out of Putin's control. Even in Crimea, where ethnic Russians predominate, many ethnic Russians were shocked by Russian armed intervention and even fled to Ukraine in horror. Dictators certainly don't listen to different opinions, what's more, Russian diplomats have always been idiots, so Putin thinks that intimidation by force can succeed. Who knows that this will unite the originally scattered Ukrainian opposition, gain unprecedented support and international support, and even lead to unprecedented international isolation. Therefore, Putin can only gradually ease the situation now and never dare to start work. He can only show the existence and encirclement, trying to intimidate and convince the Ukrainian army in Crimea.