The Peace Treaties of 1919–1920 made many territorial changes in eastern Europe, many of the influenced by the principle of ‘self-determination’ – the right of people to rule themselves.
The Treaty of Versailles – although it was mainly concerned with Germany – took land which Germany had held (or had gained by the Treaty of Brest-Litovsk when Russia accepted defeat in 1917). It gave this land to create the new nation states of eastern Europe. Land on the Baltic coast created the new nation-states of Estonia, Latvia and Lithuania. The German town of Memel was given to Lithuania. West Prussia (including the ‘Polish corridor’) and Upper Silesia were taken as part of the new nation-state of Poland, and Danzig was made a ‘free city’.
The other treaties of 1919–1920 followed the principles of the Treaty of Versailles – they punished the defeated powers, made them disarm and pay reparations, but they also took land from them to create the new nation states.
The Treaty of Saint Germain with Austria in 1919 took land from the old Austrian empire to create Czechoslovakia and (by giving it to Serbia) Yugoslavia. Austria also lost territory to Poland.
The Treaty of Trianon with Hungary in 1920 took land from Hungary and gave it to the new nation-states. Transylvania was given to Romania, and land was given to Czechoslovakia and Yugoslavia.
The Treaty of Neuilly with Bulgaria in 1919 gave Bulgarian land to Greece, Romania and Yugoslavia.
The Treaty of Sèvres with Turkey in 1920 was mainly about splitting up the Turkish empire between France and Britain, but as part of the Treaty, most of the small area of Turkey-in-Europe around Constantinople was given to Greece. In fact, however, this did not last very long – the Turks went to war with the Greeks and drove them out, and the Treaty of Lausanne 1923 gave back this land to Turkey.