However, he continued composing. Exempted, as a teacher, from military service, Martinu produced many works during the World War I, including the patriotic cantata Czech Rhapsody (1918). Although this work and two ballets, Istar (1918-1921) and Who is the Most Powerful in the World? (1922-1923), gained favorable attention. Martinu felt the need for additional training. Returning to the Conservatory, he studied composition Josef Suk, later working in Paris with Albert Roussel, whose muscular, rhythmically vigorous music eventually influenced Martinu's own. Martinu's music was well received in postwar Paris. Like many of his contemporaries, Martinu absorbed the influence of jazz, as evidenced in such works as the ballet La revue de cuisine (1927), which also incorporates South American rhythms, and the one-act opera Les larmes du couteau (The Tears of the Knife; 1928). In 1930, Martinu's constant desire to learn more led him to the music of Corelli, Vivaldi, and Bach, signaling a new concern with rhythmic continuity and contrapuntal technique.
Following the resounding success of his opera Juliette in Prague in 1938, World War II forced Martinu to flee his adopted home of Paris. After spending nine miserable months in the south of France, the composer and his wife made their way to Spain, and then to America, in the early months of 1941. For the duration of the war, Martinu lived in various cities in the Eastern United States, surviving on commissions and producing five symphonies by 1946.
Though Martinu had planned to return to Czechoslovakia after the war, injuries and health problems prevented him from traveling. After Czechoslovakia fell to the communists in 1949, it gradually became clear to Martinu that he was no longer welcome in his native land, a source of great pain to him. He eventually regained his health, however, producing such works as the Sixth Symphony (1951-1953), widely regarded as a masterpiece, two operas for television, and many chamber compositions. Martinu became an American citizen but spent much time in Europe; in 1953-5 he was based in Nice and in 1955-6 he was teaching at the American Academy in Rome. After a final New York sojourn he took up residence as the guest of Paul Sacher in Liestal, Switzerland, where he died in 1959.
Harry Halbreich's catalog of Martinu's music, to which the composer did not assign opus numbers, lists nearly 400 compositions. Well established in the repertoire, Martinu's best works confirm Martinu's status as an important twentieth century composer.