Józef Klemens Piłsudski (, 5 December 1867 – 12 May 1935) was a Polish statesman—Chief of State (1918–22), "First Marshal" (from 1920), and leader (1926–35) of the Second Polish Republic. From mid-World War I he had a major influence in Poland's politics, and was an important figure on the European political scene. He is considered largely responsible for the creation of the Second Republic of Poland in 1918, 123 years after the Partitions. Under Piłsudski, Poland annexed Vilnius from Lithuania following Żeligowski's Mutiny but was unable to incorporate most of his Lithuanian homeland into the newly resurrected Polish State.
Early in his political career, Piłsudski became a leader of the Polish Socialist Party. Concluding, however, that Poland's independence would have to be won by force of arms, he created the Polish Legions. In 1914 he anticipated the outbreak of a European war, the Russian Empire's defeat by the Central Powers, and the Central Powers' defeat by the western powers. When World War I broke out, he and his Legions fought alongside the Austro-Hungarian and German Empire's to ensure Russia's defeat. In 1917, with Russia faring badly in the war, he withdrew his support from the Central Powers.
From November 1918, when Poland regained independence, until 1922 Piłsudski was Poland's Chief of State. In 1919–21 he commanded Poland's forces in the Polish-Soviet War. In 1923, with the Polish government dominated by his opponents, particularly the National Democrats, he withdrew from active politics. Three years later, he returned to power with the May 1926 coup d'état, and became the de facto ruler of Poland. An Italian fascist ambassador to Warsaw described him as "a liberal democrat in the clothes of an old-world knight". From then until his death in 1935, he concerned himself primarily with military and foreign affairs.
For at least thirty years until his death, Piłsudski pursued, with varying degrees of intensity, two complementary strategies, intended to enhance Poland's security: "Prometheism", which aimed at breaking up, successively, Imperial Russia and the Soviet Union into their constituent nations; and the creation of an Intermarum federation, comprising Poland and several of her neighbors. Though a number of his political acts remain controversial, Piłsudski's memory is held in high esteem by his compatriots.