The Treaty of Trianon was signed with Hungary at the Paris Peace Conference on 4 June 1920. Simply put, the peace agreement saw the Austro-Hungarian Empire broken up in order to allow self-determination for the different nationalities that existed in the Empire prior to the outbreak of war.
If you want to get into the nitty gritty of things, the Treaty of Trianon transferred land from Hungary to Romania, Czechoslovakia and Yugoslavia. Romania acquired Transylvania, Czechoslovakia acquired Ruthenia and Slovakia, and Yugoslavia acquired Slovenia and Croatia.
This left Hungary as a landlocked state (pretty tragic, am I right), with a population of just 7.6 million – compared to its pre-war population of 20.9 million.