Despite the pacts, the Soviet Union's fears continued.[8] In a stance long held by Russian military theorists, control of the Baltic Sea was crucial to the defense of St. Petersburg, Russia's second-largest city[9] and the Baltic states offered a buffer zone between Russia and Germany.[10] Pursuing this strategy, the Soviet Union initiated the Winter War in Finland after that country rejected a similar Moscow-offered mutual assistance treaty.[8][11] Stalin was unnerved by German successes in Europe, since they had taken Denmark, Norway, the Netherlands, Belgium, and Luxembourg by spring of 1940.[12] According to Nikita Khrushchev, after the fall of France in May, Joseph Stalin expressed the concern that Adolf Hitler would 'beat our brains in'.[12]