Racism is fluid, and in times like this, US and UK media likes to try and spin Russians as asiatic barbarians. They invoke tropes associated with the Mongol empire and call Russians "orcs". This goes back as early as the nineteenth century when the British and Russian empires were in an intense rivalry over control of central Asia.

Whiteness isn’t fundamentally about having white skin, which is what I assume you mean by referring to Caucasian ancestry.

The simple equation of whiteness and having light skin might be approximately true inside the US because of the whole slavery thing. The influx of African slaves prompted the Europeans to reconcile their ethnic differences on that superficial basis alone. But that didn’t erase racism between nominally white ethnicities. For example, Italians and Irish people have not always been accepted as white in the US.

The basis of racism is the categorization of certain peoples into pseudo-scientific races, and Europe has a long history of excluding Slavic peoples as a distinct race from the “civilized” western Europeans.