Keeping things concise I will just mention a few examples.
Hitler and his Nazism, the United States and their enslaving and discrimination against blacks and, removal of most of Native American Land. The Europeans dividing Africa as though it is a piece of property (imperialism). And any other things that are not mentioned here.
Denmark, Britain and Sweden? Absolutely. Germany and Italy not so much. It's those countries I mentioned that have done some pretty horrid and horrible things to their subjects. Italy and Germany never took it upon themselves to rob and steal and kill their subjects. Danish, Brits and Swedes were some of the most brutal conquerers on the planet.
I think it is clear in history that whites are not the only supremists. Middle Easterners enslaved many white people, the Mongols believed they were superior to others and gave each other positions of power over others. The Japanese once expelled all of the white people from Japan because they were "Infecting the Japanese people". We have just seen a bigger worldwide affect of white supremacy, because they controlled much of the world and remained in relative dominance in those areas for years. This occurred simply because they happened to have access to the recourses and war, not trade, isolation necessary to safely industrialize and develop a higher grade of weaponry.
Humans are tribal creatures. We naturally organize ourselves into groups for protection, safety, acquisition of resources, etc. Historically, tribes have killed or enslaved members of competing tribes. This behavior nothing to do with being white. Whites do not have a monopoly on ethnocentrism. Still, since the question is "Are whites historically supremacists", I am forced to agree.
Although slavery does exist, it was not just about whites enslaving blacks. The slave commerce was reciprocal. Black slaves were sold in countries with a mostly white population, and white slaves were sold in countries where the majority was black. It was equally common.
Of course, we only hear half the story (blame lobbies), but it does not mean the other half never existed.
This also invalidates the whole concept of affirmative action, as giving that both races were enslaved, there is nothing to compensate.
People tend to forget that some historical details, like, say, while we did massacre some American-Indians first, they also massacred white men, women, and children, including, of course, scalping them alive, and, sometimes, they attacked first.
This is not a one sided fight. Both sides have dirty secrets and black eyes. People like to point to the Crusades as proof of white men attacking native people, but again, forget that the Islamic Warriors often marched through Israel conducting forced conversions (ie: the sword, or Islamic conversion) and Jewish massacres.
If you study hard you will see that it isn't all white people all the time like some would have you believe and honestly I am getting really sick of white man blame. It is racist and ignorant and not what men like Martin Luther King Jr. Saw in their dreams.
In fact, the KKK is rumored to be rather picky about such things. Catholics, despite being White, just aren't welcome. They never have been. Jews are not welcome. Immigrants, though most of the nation has immigrants in their ancestry, are frowned upon. During the 1960's, 3 Civil Rights workers were killed in Mississippi by the Klan. Two were White and one was Black. Being White, historically, doesn't get you an invitation into such hate groups. An individual's beliefs, for lack of a better way to describe it, carries more leverage than skin color.
This is sort of a "yes or no" answer. Because if you look at white history, yes they have a history of oppressing other races and their own people, but pretty much all races have similar history. If you think of Asians think of say the Rape of Nanking in WWII with the Japanese, or all the chinese dynasties and how they weren't the kindest of people to those who were "different." If we're talking about Blacks, then you have the Moors who were Black Muslims they enslaved white people, then there's the history of man constantly enslaving others that looked like him. Many Blacks for example enslaved blacks, whites enslaved whites, the Indians enslaved Indians, slavery has happened all throughout history with self-inflicted slavery and racial slavery. So it's hard answer this question accurately when it's termed like this, but that's my answer.