The Babel Proclamation was issued by Iowa's Governor William L. Harding on May 14, 1918. Harding took the Anti-German sentiment in the wake of World War I further than any other state.
According to this proclamation, "only English was legal in public or private schools, in public conversations, on trains, over the telephone, at all meetings, and in all religious services."
References
Babel Proclamation Wikipedia(Text) CC BY-SA