Victoria Nuland, a senior State Department official responsible for US policy on Ukraine has been ranting before Congress about Russia’s involvment in Ukraine. The conflict is “manufactured” in the Kremlin which has sent “thousands and thousands” of personnel and tanks. 
So, of course, the US must now arm Kiev. Apparently there is almost total support for Nuland in Congress. (But the Western press will no doubt not describe them as “fawning” as they do when Russian law-makers rally around Putin).
This is the line that they have told themselves and thus believe: Russian aggression against the territorial integrity and sovereignty of Ukraine. As this web site has discussed in multiple recent posts