Soviet fears of a German invasion, further aggravated by German expansion to the East, such as the ultimatum to Lithuania in March 1939, as a result of which the nation was forced to cede its most industrially developed region, Klaipėda, to the Reich. The Soviet Union signed the Molotov–Ribbentrop Pact with Germany in August 1939, in part as an attempt to delay the possibility of invasion. Germany shortly initiated World War II by invading Poland on September 1. Lithuania at first was found to be in Nazi Germany's sphere of influence according to the secret protocol of Molotov–Ribbentrop Pact, but later German–Soviet Boundary and Friendship Treaty of September 28, divided large portions of northeastern Europe between the two powers, and assigned Lithuania to the Soviet sphere of influence. A Lithuanian delegation was invited to Moscow, where it signed the Soviet–Lithuanian Mutual Assistance Treaty on October 10, 1939. According to the treaty, the Soviet Union would cede a portion of the Vilnius Region, including the important city of Vilnius, which it had gained during the invasion of Poland, to Lithuania in exchange for the right to station up to 20,000 (the original bargaining point was 50,000) Soviet troops in Lithuania on a permanent basis. Official Soviet sources claimed that the presence of the Soviet military was necessary to strengthen defenses of a weak nation against possible attacks by Nazi Germany. In reality, it was the first step toward the eventual occupation of Lithuania and was described by The New York Times as a "virtual sacrifice of independence." Despite the pacts, the Soviet Union's fears continued. In a stance long held by Russian military theorists, control of the Baltic Sea was crucial to the defense of St. Petersburg, the Soviet Union's second-largest city, and the Baltic states offered a buffer zone between the Soviet Union and Germany. Pursuing this strategy, the Soviet Union initiated the Winter War in Finland after that country rejected a similar Moscow-offered mutual assistance treaty. Stalin was unnerved by German successes in Europe, since they had taken Denmark, Norway, the Netherlands, Belgium, and Luxembourg by spring of 1940. According to Nikita Khrushchev, after the fall of France in May, Joseph Stalin expressed the concern that Adolf Hitler would 'beat our brains in'. The political situation in Lithuania, however, remained stable between October 1939 and March 1940. The Soviets did not interfere with Lithuania's domestic affairs and the Soviet