http://en.wikipedia.org/wiki/Worldwar_series

World War series is a fan name given to a series of Harry Turtledove's alternate history science fiction novels; it includes the Worldwar tetralogy, as well as the Colonization trilogy, and the novel Homeward Bound. Its settings range from 1942 to 2031.

Interplanetary War

The saga opens with a conquest fleet of aliens arriving on Earth at the time of World War II. These aliens have planned invasion for centuries, based on the findings of a probe that examined Earth at the time of the Middle Ages. "The Race" expected to find life and technology on Earth (to them known as Tosev 3) the same as when their probe found it, as the Race themselves develop very slowly. The first series deals with the war, diplomatic negotiations, and the establishment of Race control over most of the world.

Worldwar Tetralogy

In the Balance (1994)
Tilting the Balance (1995)
Upsetting the Balance (1996)
Striking the Balance (1996)

Uneasy Peace

The second series of novels, set in the 1960s, deals with the interaction between surviving humans and the Race. It opens with the arrival of the colonization fleet. The series ends with the defeat of Nazi Germany and the establishment of a permanently manned US space-station in the asteroid belt. Part of this series focused on the Reich-Race War of the mid-60's, when the Greater German Reich and the Race fought a nuclear war. The Germans lost, and were forced to let France become an independent nation again.

Colonization Trilogy

Second Contact (1999)
Down to Earth (2000)
Aftershocks (2001)

Interstellar

The final novel in the saga deals with humanity reaching the Race's homeworld, "Home" (Tau Ceti II).

Homeward Bound (2004)

Much thanks to the OP!