The answer is The Civil War
Article II, Section 2 of the Constitution declares that the president "shall have Power, by and with the Advice and Consent of the Senate, to make Treaties, provided two thirds of the Senators present concur." But it doesn't say anything about the power to break treaties.
Nobody is online for history so ill try to help you. I think its suburbs but I could be wrong if you think its something else you are probably right.
