Identify an idea or event from twentieth or twenty-first century history that was influenced by stereotypes.?

The internment of the Japanese Americans in WWII. Many Americans, including 'well meaning' ones were sure the Japanese Americans, even those born in the US would be loyal to the Emperor of Japan, a 'god' in Japan and not to the US government. This was basically not true, but caused many thousands of innocent people to spend most of the war in camps, better than German concentration camps, but still a terrible thing.