The death of his father in 1932 severed Hartung's last bonds with Germany. He was rejected from Nazi Germany on account of being a 'degenerate', because his painting style was associated with Cubism – an art movement incompatible with Nazi Germany's ideals. In 1935 when he attempted to sell paintings while visiting Berlin, the police tried to arrest him. He was able to flee the country with the help of his friend Christian Zervos.
In December 1939, he became a member of the French Foreign Legion. He was closely followed by the Gestapo and arrested for seven months by the French police. After they learned he was a painter, he was put in a red cell in an attempt to disturb his vision. After being released he rejoined the Legion to fight in North Africa, losing a leg in a battle near Belfort. He earned French citizenship in 1945, and was awarded the Croix de Guerre.
Hartung's freewheeling abstract paintings set influential precedents for many younger American painters of the sixties, making him an important forerunner of American Lyrical Abstraction of the 1960s and 1970s. He was featured in the 1963 film documentary "School of Paris: (5 Artists at Work)" by American filmmaker Warren Forma.In the 1970s, Hartung and Anna-Eva Bergman remarried. He died on 7 December 1989, in Antibes, France.Wikipedia