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.