To be technically correct, France and Britain had a well-publicized mutual-defense treaty with Poland (among others), and when Germany attacked Poland (knowing full well of the alliance) the signatories had no choice but to declare war. In fact, Britain and France gave Germany an ultimatum to pull out of Poland before initiating hostilities.
Furthermore, Germany actually didn't even bother declaring war on Poland first before launching their first attacks.
In other words, this was not a Pearl Harbor, Germany knew full well what they were getting into. Yeah, Britain and France were not iconic examples of bully victims, but if you're suggesting Germany is, you'd be mistaken.
The analogy is that while the enemy wasn't beating him up and "occupying" him as badly as they did others, he still had to deal with a bit of a blitz and ended up using his ingenuity to break their enigmatic code, develop a radar against future attacks, and ultimately develop a world class force of his own to contend with them.
Of course, the analogy breaks down around there unless he made friends with an even-less-bullied kid and another bully who suddenly became outcast and bullied in his own right, and worked together to defeat the other bullies once and for all.
Off topic: actually France and Britain attacked and declared war on Germany. So probably neither works as a metaphor for a victim of bullying.