Certainly in the United States, whiteness has very often defined itself against blackness. To be white was not to be black. Everyone else was left in between and there was room for tactical accomodations. If you go back to the 1910s and 1920s, you see a series of legal cases that reached the Supreme Court where, in quick succession, you have a Japanese person say 'well I count as white' and the Supreme Court says you have to be Caucasian to count as white. Then a person from India sues and says 'I count as white because I come from Caucasian stock' and the Supreme Court says 'no we don't mean Caucasian in that sense, we mean what an ordinary person would think of as Caucasian.' You get this circular logic that defines whiteness from the very beginning.