Watch on the Rhine

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Watch on the Rhine is a military science fiction novel by John Ringo and Tom Kratman, the seventh entry in Ringo's Legacy of the Aldenata series.
The novel focuses on the invasion of Europe by the alien Posleen, with an emphasis on Germany. Part of the technology brought to humans by the Galactics is the ability to rejuvenate old soldiers, so that countries can draw on their combat experiences. In Germany, this leads to the controversial decision to reactivate the Waffen-SS.
The book's title is a reference to the 19th-century German nationalist song "Die Wacht am Rhein".