What political events in the 1900s made European countries believe they needed work together?

The early 20th century saw a series of political events that underscored the necessity for cooperation among European countries. The rise of nationalism and militarism led to tensions, exemplified by the Balkan Wars and the assassination of Archduke Franz Ferdinand in 1914, which precipitated World War I. The devastation of the war and the subsequent Treaty of Versailles highlighted the need for diplomatic engagement to prevent future conflicts, ultimately leading to the establishment of the League of Nations in 1920. Additionally, the rise of totalitarian regimes in the 1930s and the onset of World War II further emphasized the importance of collective security and collaboration among nations to ensure peace and stability in Europe.