In the brief hiatus between the Nazi–Soviet Pact and the German invasion of the Soviet Union, Stalin scrambled to consolidate his hold over his recently acquired borderlands and to advance additional demands for strong points along the periphery of the USSR. Contrary to his intentions, the consequences were disastrous. His brutal treatment of the populations in the newly acquired borderlands provoked hostility to Soviet power and increased social instability. His utter lack of sensitivity to the reaction of the powers, both Axis and Western, brought him to the brink of war with France and Britain over Finland and possibly even accelerated the Nazi attack. It was very much the style of a Commissar of Nationalities. Subjugating and integrating the borderlands was for him the best guarantee of state security. The opinion of the outer world counted for very little.

If Stalin had long been preparing an accommodation with Hitler, his actions after August 1939 gave no evidence of having used the time well. Plans for the partition of Poland had been drawn up hastily. Stalin quickly had second thoughts about incorporating large numbers of Poles into the Soviet sphere. The agreement over Poland had to be renegotiated almost at once. Stalin's advance into the other territories in his sphere was marked by confusion, haste and enormous errors of judgment. He had not even taken the trouble to prepare the ground politically. On the contrary, he had made his task of absorbing the borderlands more difficult for himself. In the case of the Baltic states, Western Belorussia and Western Ukraine he had only recently decimated the ranks of local Communist parties. In the case of Finland he improvised a political solution that proved to be a complete fiasco.

The Baltic and Danubian frontiers

Shortly after the collapse of Poland, Stalin belatedly decided that it would be preferable to turn over all of the heavily ethnic Polish provinces to Hitler's tender mercies in exchange for Lithuania. The original demarcation of spheres had divided Warsaw in half. The revised German–Soviet Boundary Treaty of September 28, 1939 brought the Soviet frontier back to the Bug River.