There are 5 books in the Worldwar series. The newest book is Homeward Bound which came out in 2005.

1
In the Balance

Book cover of In the Balance
War on earth erupted in every corner of the globe. Then the real enemy came. Inhuman invaders who were unstoppable, their technology far beyond our reach, their simple goal to claim Earth for the Empire. Here is a saga that covers all the Earth, and beyond, as mankind--in all its…
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2
Tilting the Balance

Tilting the Balance
NO ONE COULD STOP THEM--
NOT STALIN, NOT TOGO, NOT CHURCHILL, NOT ROOSEVELT . . .
The invaders had cut the United States virtually in half at the Mississippi, vaporized Washington, D.C., devastated much of Europe, and held large parts of the Soviet Union under their thumb.
But humanity would…

3
Upsetting the Balance

Book cover of Upsetting the Balance
Russia, Communist China, Japan, Nazi Germany, the United States: they began World War II as mortal enemies. But suddenly their only hope for survival—never mind victory—was to unite to stop a mighty foe—one whose frightening technology appeared invincible.

Far worse beings than the Nazis were loose. From Warsaw to Moscow…

4
Striking the Balance

Striking the Balance

When alien beings armed with devastatingly superior military technology and bent on conquest invaded earth, Allied and Axis forces were already engaged in a bloody conflict - the Second World War - that spanned the whole globe. Suddenly, humans had to stop fighting each other and unite against this deadly…

5
Homeward Bound

Book cover of Homeward Bound
The twentieth century was awash in war. World powers were pouring men and machines onto the killing fields of Europe. Then, in one dramatic stroke, a divided planet was changed forever. An alien race attacked Earth, and for every nation, every human being, new battle lines were drawn. .

HOMEWARD…