I'm about 3/4 done with an interesting book. It's part of the Posleen invasion series, books that describe a massive (on the order of multi-million quickly reaching billion) alien soldier invasion of Earth.
Times are desperate, and the stakes are high. It takes place in 2004, and some friendly aliens have given some assistance in the form of technology, but it's still a real challenge because of the swarms of alien infantry. As a result, every nation is building the biggest armies they can, conscripting everyone possible and trying to route as many civilians to underground fortresses.
One of the pieces of technology is rejuvenation therapy. Anyone with military background, no matter how old, can be brought back into fighting trim. The military is full of WWI and WWII vets, plus veterans of all other wars that were still alive. Even the senile can be recovered from nursing homes.
This book, "Watch on the Rhine", chronicles the invasion of Europe. With the Whermacht as defanged and declawed as it is today, turning it into an effective fighting force is very challenging. Lacking any other options, the german government has turned to re-activating the last group they wanted: The SS. Thousands of retired SS soldiers plucked from rest homes and turned into 20 something soldiers again, forming the training backbone for many brigades.
It's a good read, and funny to see the political ramifications of putting those folks in positions of military power.
John Ringo and Tom Kratman are the authors.