 During his years in Cambridge, from 1911 to 1913, Wittgenstein conducted several conversations on philosophy and the foundations of logic with Russell, with whom he had an emotional and intense relationship, as well as with Moore and Keynes. He retreated to isolation in Norway, for months at a time, in order to ponder these philosophical problems and to work out their solutions. In 1913 he returned to Austria and in 1914, at the start of World War I (1914–1918), joined the Austrian army. He was taken captive in 1918 and spent the remaining months of the war at a prison camp. It was during the war that he wrote the notes and drafts of his first important work, Tractatus Logico-Philosophicus. After the war the book was published in German and translated into English.