If you haven't read John Ringo's A Hymn Before Battle and Gust Front, they set the background for the war in Watch On The Rhine. Reading the 3rd and 4th Ringo books in the Posleen war series aren't as necessary. In summary there's several waves of evil alien invasion coming, and some warning and technology from some supposedly friendly aliens is the only help we'll get.
In WOTR, the German Chancellor views the aftermath of the bloody battles in Northern Virginia described in Gust Front. Realizing the brutal nature of the impending invasion, he decides with the reluctant support of his government to use alien rejuvenation technology used to recreate an SS combat unit. In this war no negotiation is possible and every tool available must be used.
The story centers on a super-tank crew of 2 rejuvenated WW2 veterans and several young recruits. The tank commander names the Tiger III after his long dead Jewish wife. Several flashbacks occur throughout the book detailing his past history of WW2 combat, survival, love shared with a Jewish woman, and witnessing deathcamp horror. The story progresses through their training, friction with civilians violently opposed to a reinstated SS unit, and brutal combat with the aliens.
This isn't a Walt Disney portrayal of good vs evil. This is more of a "use one evil to fight another evil". There are no magic pills or roads to military victory: their battlefield successes are the result of harsh, realistic training, close comradeship, cunning tactics, and careful weapon design influenced by early combat experience against the aliens. Unlike Disneyish entertainment, the SS veterans relish combat. They pass on their unique trade skills to their new recruits like parent wolves training their cubs.
Similarly, they have no patience or compassion for soldiers or civilians who flee their duty or get in their way. Their ruthlessness will become necessary against the alien Posleen invaders who reveal a shocking new battlefield tactic.The research into both WW2 history and German language and culture is impressive. You can easily visualize the scarred veterans leading their recruits in verboten Third Reich marching songs and flaunting their black uniforms. At the same time the authors communicate well the pride of these soldiers in themselves and their brothers-in-arms.
The story centers heavily on the super-tanks created for fighting the Aliens. If you liked reading Keith Laumer's Bolo books or Hammers Slammers by David Drake, you'll love this story.
If you like military science fiction, written with a hard edge of reality of a desperate situation with characters both young/rejuvenated/old you can identify with, this is a very good read.