After WWI what was used as the principal criteria for identifying ethnic groups in Europe?

The self-determination of states, the principle that peoples, based on respect for the principle of equal rights and fair equality of opportunity, have the right to freely choose their sovereignty and international political status with no interference, developed throughout the modern period alongside nationalism.

During and especially after World War I, there was a renewed commitment to self-determination and a major influx of new states formed out of the collapsed empires of Europe: the German Empire, Austro-Hungarian Empire, Russian Empire, and the Ottoman Empire.

Many new states formed in Eastern Europe, some out of the 1918 Treaty of Brest-Litovsk, where Russia renounced claims on Poland, Finland, Estonia, Latvia, Belarus, Ukraine, and Lithuania, and some from the various treaties that came out of the Paris Peace Conference of 1919.

These new countries tended to have substantial ethnic minorities who wished to unite with neighboring states where their ethnicity dominated (for example, example, Czechoslovakia had Germans, Poles, Ruthenians and Ukrainians, Slovaks, and Hungarians), which led to political instability and conflict.

The dissolution of the Ottoman Empire became a pivotal milestone in the creation of the modern Middle East, the result of which bore witness to the creation of new conflicts and hostilities in the region.