Following the Nazi invasion of the Soviet Union, which cut off their supply of tungsten metal, Germany sought tungsten from Portugal. Salazar attempted to limit their purchases, and in late 1941, German U-boats attacked Portuguese ships. In January 1942 Salazar signed an agreement to sell tungsten to Germany. In June 1943, Britain invoked the long-standing Anglo-Portuguese Alliance requesting the use of the Azores, to establish an air force and naval air base. Salazar complied at once. The Allies promised all possible aid in the event of a German attack and guaranteed the integrity of Portugal's territorial possessions. In 1944, Portugal declared a total embargo of tungsten shipments to Germany. Germany protested but did not retaliate.