The major event that led to the establishment of the Republic of Turkey was the end of World War I and the subsequent Turkish War of Independence, which occurred from 1919 to 1923. After the Ottoman Empire's defeat, the Allies partitioned its territories, leading to widespread dissatisfaction among Turkish nationalists. Under the leadership of Mustafa Kemal Atatürk, a movement arose that successfully expelled foreign occupying forces, culminating in the 1923 Treaty of Lausanne. As a result, the official proclamation of the Republic of Turkey occurred on October 29, 1923, marking a new secular and nationalistic era for the country.