I could never figure out why the Uptimers didn't just drive to Berlin and sail into Tokyo. They showed the ability to attack the Axis forces with pretty much impunity. I understand why they went after the POW camps, they felt really sorry for the POWs, but they could have gotten the same effort and ended the war three years earlier, by nuking Tokyo and landing the 82nd MEU in Germany and driving them to Berlin.
They didn't have nukes, so they couldn't have nuked Berlin. Also, this was discussed in the first book. Sure, they could have ended the war quickly, but then they would be left sitting around, an existential and military threat to America. By liberating the POW camps, they show the 'temps that they care about what's going on, and build a 'reserve of goodwill'.