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.