Following the Soviet occupation of eastern Poland, of the Baltic states and of Bessarabia and Northern Bukovina in 1939–1940, Stalin insisted on the occupation of every fold of the newly Sovietized territories; this move westward positioned troops far from their depots, in salients that left them vulnerable to encirclement. As tension heightened in spring, 1941, Stalin desperately tried not to give Hitler any provocation that Berlin could use as an excuse for a German attack; Stalin refused to allow the military to go on the alert – even as German troops gathered on the borders and German reconnaissance planes overflew installations. This refusal to take necessary action was instrumental in the destruction of major portions of the Red Air Force, lined up on its airfields, in the first days of the German-Soviet war.