Colonization: Down to Earth is the second novel in the Colonization series, and the sixth dealing with the Race's invasion of Earth. It is preceded by Second Contact, and followed by Aftershocks.
Following the nuclear attack on the colonist ships in Second Contact, the Race continues to try to find the responsible nation, along with the purpose of the Lewis and Clark, a large space station launched by the United States. At the same time, the range animals brought by the Race colonists begin to spread into the human nations, causing ecological trouble and causing conflicts between them. President Earl Warren and Vice President Harold Stassen are reelected to office.
Meanwhile, in the Soviet Union, General-Secretary Vyacheslav Molotov tries to play his country's enemies, Nazi Germany and the Race, off each other. His efforts unravel when Heinrich Himmler, the Führer, dies unexpectedly at the end of 1964. He is finally succeeded by Ernst Kaltenbrunner, who carries out Himmler's planned invasion of Poland in 1965, initiating a nuclear war between Germany and the Race.