Thoughts on Harry Turtledove's Worldwar series #1 "In the Balance",
#2 & #3.

I quite enjoyed this book, and the 2nd got better, and
IMHO, The 3rd even better! The 4th was quite fine.

It starts in Spring 1942 with WWII in quite full force.
The aliens have around what we might consider 1995
military capability. Turtledove offers the perspective of
about 60 individuals, with sections from 3 to 12 pages long.
So, if bored or unhappy with a section, it will soon change!

"In the Balance (Worldwar #1)"
by Harry Turtledove

https://www.goodreads.com/book/show/101527.In_the_Balance?from_search=true&from_srp=true&qid=QOUy0ilbL8&rank=3

https://www.amazon.com/Balance-Worldwar-Book-One-ebook/dp/B000FBFOQY
In the Balance (Worldwar, Book One) (Worldwar Series 1)
by Harry Turtledove

"Suppose Roosevelt, Stalin, Churchill, Hitler, and Hirohito had
united to conquer an even greater foe?
No one could top their power—not the Germans, not the Japanese,
not the Russians, not the United States.
From Pearl Harbor to panzers rolling through Paris to the Siege
of Leningrad and the Battle of Midway, war seethed across the
planet as flames of destruction rose higher and hotter.

And then, suddenly, the real enemy came.
The invaders seemed unstoppable, their technology far beyond
human reach. And never before had men been more divided. For
Jew to unite with Nazi, American with Japanese, and Russian
with German was unthinkable.
But the alternative was even worse.
As the fate of the world hung in the balance, slowly,
painfully, humankind took up the shocking challenge. . . ."

"Editorial Reviews
From Publishers Weekly
This intelligent speculative novel depicts an alternate history
in which, at the height of World War II, Earth is attacked by
alien beings with weapons far more destructive than any possessed
by the Allied or Axis forces. Turtledove ( The Guns of the South )
gives a surprisingly convincing flavor to the time-worn story of
warring nations uniting to repel extraterrestrials; his human
characters, both actual and invented, ring true as they struggle
to trust each other after years of enmity, and although the alien
threat has a B-movie feel, he makes an effort to portray the
invaders sympathetically as well. The first in a projected series,
the book ends where it began: in and around a battle. The smooth
writing is marred only by slightly overdone dialogue for real-life
figures like General Patton. The historical details, especially
those concerning the weapons and methods available in the 1940s
to defend Earth, are accurate and well rendered."

and also
https://www.amazon.com/gp/product/B000FBFOQE?notRedirectToSDP=1&ref_=dbs_mng_calw_1&storeType=ebooks
Tilting the Balance (Worldwar, Book Two)

https://www.amazon.com/gp/product/B00G5KGNUU?notRedirectToSDP=1&ref_=dbs_mng_calw_2&storeType=ebooks
Upsetting the Balance (Worldwar, Book Three)

https://www.amazon.com/gp/product/B000FBFOR8?notRedirectToSDP=1&ref_=dbs_mng_calw_3&storeType=ebooks
Striking the Balance (Worldwar, Book Four)

and for those that prefer a wiki:
https://en.wikipedia.org/wiki/Worldwar:_In_the_Balance#:~:text=Worldwar%3A%20In%20the%20Balance%20is%20an%20alternate%20history,the%20Colonization%20trilogy%20and%20the%20novel%20Homeward%20Bound.

https://en.wikipedia.org/wiki/Worldwar_series#List_of_Characters