The word "Poland" was written officially for the first time in 966. In 1569, Poland formed a strong union with Lithuania called the Polish-Lithuanian Commonwealth. At some point in its history, it was the largest state in Europe and became very influential. Much of the territory that now makes up Central European states used to belong to that Commonwealth. Eventually, after a slow decline, the Commonwealth collapsed in 1795. Poland regained its independence in 1918 after World War I. In 1921, Poland defeated Soviet Russia in the Polish-Soviet War that started in 1919.