In August 1939 Tarski traveled to the United States to attend a congress of the Unity of Science movement (see Vienna Circle). World War II broke out soon after that, leaving Tarski no option but to stay in the States. He spent the war years separated from his family, forced to remain in Poland. In this period he held several temporary university positions, at Harvard University, the City College of New York, the Institute for Advanced Study at Princeton, and the University of California at Berkeley, where he was eventually given tenure in 1945 and a professorship in mathematics in 1948. Maria, Ina and Jan were able to join him in Berkeley in 1946.