The misleading claim that the Baltics were ceded to the USSR before Yalta ignores the earlier disagreement between Roosevelt and Churchill about the Baltics.
Before officially ending the War, Churchill wanted to prolong the war 6 months in order to roust Russia from the Baltics. This would have meant that troops would move at least from Poland to Greece in a liberating march. Roosevelt strongly opposed Churchill. He said that the US citizenry would not tolerate 6 more months of war.
In fact there was no question of war or of spilling blood. By a grand victory march to the Mediterranian the Western Allies would have established the overthrow of the Third Reich in the Baltics and enforced national boundaries. Russia could not have opposed this move militarily. However, just as in East Germany, Stalin strongly objected to the Allied liberation of the Baltics based on Russia's geopolitical interests.
In fact the US citizenry was chomping at the bit to free the Baltics in a grand victory parade of freedom and democracy. Many Americans hailed from these countries and had strong feelings against Russian occupation. The American people understood that the war was about freedom from opression.
I remember my German and British grandparents were depressed for years over America's failure to liberate these nations so sorely opressed by Hitler. There was a great sense of betrayal here in the US over the failure of the Allies to liberate those nations for whom we went to war.
The left has a deeply rooted vested interest in rewriting history. Their primary impetus seems always to soften criticism of Marxist states and make Utopian claims about socialism. The left equivocates in the face of ghastly realities of mass murder and all forms of vicious opression by Marxist tyrants. ("They have not tried pure Communism MY way," remains the idiotic rejoinder.)
Roosevelt's chief negotiator, Alger Hiss, was a Russian plant, a communist and a traitor. That traitor manipulated Roosevelt into supporting 50 years of tyranny in the Baltics.
Roosevelt was, after all, a dying, weak president with no heart left for shedding American blood. That may be the most positive rationale for this remarkable and shocking act of cowardice.