Broadly speaking, the Soviets got to keep this Finnish territory along with territories on the Baltic, prewar Germany, Romania, Poland, Czechoslovakia, and Hungary for two interrelated reasons.

One, it certainly helped the Soviets that a number of these countries were part of the Axis powers. The Romanian, Hungarian, and Finnish governments all allied with Germany and participated in the attack on the USSR. This aided the Soviet case for territorial readjustment internationally because there was relatively little sympathy for states that had made common cause with Hitler. Even though Finland did not declare war on the US, its alliance with Germany led to a drawdown in diplomatic relations between the two countries and a distinct chill in US-Finnish relations. The German alliance had trapped Helsinki into a diplomatic cul-de-sac in which it found few potential non-Soviet patrons to get it out of the Axis alliance. This persona non grata status was repeated with all of the minor Axis powers to some degree where they had very little room to maneuver after it became clear Germany was going to lose the war. And while the diplomacy of the minor Axis was now constrained, the exact opposite process happened with Stalin. The Soviets found they could negotiate readjustments or compel surrender terms by fiat. Moreover, the fact that these countries' governments were part of the Axis provided a justification for Soviet replacement by a friendlier government. The global nature of communism meant there were always a ready cadre of communist exiles who could follow in the wake of the Red Army and act as a new provisional government that would toe the line from Moscow. Finland avoided this fate by being among the first Axis powers to make a deal with the Soviets and thus got a fair number of political concessions, but the ceded geographic territories were the price to pay for these concessions.

The other reason, and one that arguably had the greater impact, was that the Red Army was strong enough to pretty much impose these borders by force if necessary. The Soviets had boots on the ground in Eastern Europe and Anglo-American diplomats knew this. Negotiations at Yalta and later Potsdam were tense, but there was realistic assessment that this was a region of the world where the Soviets already had a preponderance of military power. Short of war, there was relatively little that the Anglo-Americans could do to rectify this situation. The Anglo-American dilemma was that in order to keep Stalin involved in diplomacy for both the war against Japan and the postwar period, they had to sate these immediate territorial demands.

The Anglo-American response to Soviet territorial aggrandizement has attracted more than its fair share of critics both at the time and since. Notions of the "betrayal at Yalta" were quite common in Eastern European emigre groups as well as various anticommunist ideologues during the Cold War. While some of this critique has a point, the postwar Soviet occupation led to dictatorships, it can go too far. The Anglo-French alliance may have went to war in 1939 in response to Germany's invasion of Poland, but they did not go to war for Poland. The Polish alliance was more about drawing a line against Hitler's expansionism than it was about protecting the rights of Poland. This was emblematic of the fact that Eastern Europe lay very low on the list of priorities for Anglo-American diplomats and leaders. Not only had their participation in the Axis cost them in terms of prestige, there was a sense among some Anglo-American opinion that Eastern Europe was the natural sphere of influence for Soviet power. One example of this was that the emigre former head of the Russian 1917 Provisional Government Alexander Kerensky actually argued in editorials and letters in 1944/45 that Soviet aggrandizement in the Baltic was a natural redress of the First World War. Even though Kerensky was no fan of Stalin or the USSR, he was a proponent of expansion. So charges of betrayal often project a sympathy for Eastern Europe that just was not present among Anglo-American leaders.

The other problem with the betrayal charge was that there was no real viable option to dislodge the Soviets from Eastern Europe to restore the 1939 borders. Stalin knew the Red Army gave him a valuable card and he pushed his advantage. Refusing to acknowledge these borders was a quixotic task and could lead to a darker road of a renewed war. Such a prospect was not one any Anglo-American leader relished and even the least-worst option of diplomatic isolation had the potential to spiral into another conflict.