In hindsight, I do wonder if Stirling was making a point about the Cold War.Marching Through Georgia is the most book for teen boys book I ever read, and as a teen boy at the time I enjoyed it a treat, but it didn't leave me with much of an urge to read what my friend assured me were the more gritty serious followup stories.
On one hand, we have the Nazis – no other explanation required – and on the other we have a force that appears to be slightly more affable than Hitler, but is equally as dangerous in the long run if not more so. The historical USSR was often just as dangerous as the Nazis and perhaps more so in many ways – the novel depicting the US allying with the Draka might easily be a stand-in for the alliance with Stalin, in which the US missed a great many opportunities to curtail Soviet power until it was too late.
Actually, that might have been the point all along.
Stirling wrote Marching only a few years (not many, IIRC) before the Fall of the Berlin Wall, but that caught many people by surprise. The USSR looked invulnerable until it suddenly was not.
Of course, from a real-world point of view, the existence of a powerful and aggressive state to the south would deter Hitler from any adventures east …
… Or am I just overthinking it <grin>.