In August 1939, the Soviet Union and Nazi Germany signed the German–Soviet Nonaggression Pact and its , dividing Central and Eastern Europe into spheres of influence. Lithuania was initially assigned to the German sphere, likely due to its economic dependence on German trade. After the March 1939 ultimatum regarding the Klaipėda Region, Germany accounted for 75% of Lithuanian exports and 86% of its imports. To solidify its influence, Germany suggested a German–Lithuanian military alliance against Poland and promised to return the Vilnius Region, but Lithuania held to its policy of strict neutrality. When Germany invaded Poland in September 1939, the Wehrmacht took control of the Lublin Voivodeship and eastern Warsaw Voivodeship, which were in the Soviet sphere of influence. To compensate the Soviet Union for this loss, a secret codicil to the German–Soviet Boundary and Friendship Treaty transferred Lithuania to the Soviet sphere of influence, which would serve as the justification that enabled the Soviet Union to occupy Lithuania on June 15, 1940 and to establish the Lithuanian SSR.