 Poland was to be partitioned in the event of its "political rearrangement"—the areas east of the Narev, Vistula and San Rivers going to the Soviet Union while Germany would occupy the west.Lithuania, adjacent to East Prussia, would be in the German sphere of influence, although a second secret protocol agreed in September 1939 assigned the majority of Lithuania to the USSR.World War II began with the invasion of Poland, an important regional ally of Estonia, by Germany.