In 1909 the National Association for the Advancement of Colored People was founded by African-American activist W. E. B. Dubois and a group of Jewish Americans. Why did Jewish Americans historically ally themselves politically with African Americans?

There was (at least) one very important similarity between the Black and the Jewish community in the US: they were both discriminated by the white majority.

So by abolishing discrimination against the Black people, the Jewish population also fought for their own cause; for the end of discrimination.