In the first half of the twentieth century, the Mongolian People's Republic had a special place in world affairs as the Soviet Union's only satellite state. Following the Russian Revolution of 1917, the borders of Russia in Europe had receded, and the newly independent states in the Baltic region and Eastern Europe were for the most part suspicious of Soviet Russia and hostile towards its political intentions. Soviet–Japanese relations, officially established in 1925, were plagued from the start by a legacy of distrust left by the earlier Japanese intervention into Siberia. The Japanese invasion of Manchuria in September 1931, which soon brought Japanese troops of the Kwantung Army to the borders of the Mongolian People's Republic, aroused great alarm in Moscow.