The novel first introduces with what actual historical events happened in our timeline and then tells of the point of divergence of historical events before it begins its story.
In our timeline, in April 1945 the Allied forces in Europe, under the command of General Dwight Eisenhower, halted all further advance into Nazi Germany at the Elbe River while the Red Army battled the surviving German divisions on its way to Berlin.
However, in the alternative timeline, instead of halting the Allied advance into Germany, new US President Harry S. Truman authorizes the US Army to continue across the Elbe and head for Berlin to bring a quick end to the war to guarantee the West's share of the to-be divided German capital with their forces in the city. However, Soviet Premier Joseph Stalin, despite the agreed terms of dividing Berlin and Germany with the Western Allies, wants to take Berlin for himself, on the grounds that the Soviet Union best deserved to conquer its archenemy's capital, after the unparalleled brutality of the Eastern Front. He even goes as far as to order the Soviet Army to attack any US forces on sight if they ever get near Berlin to intimidate the West into leaving Berlin to the Soviet Union.