Why Britain went to war over Belgium and Poland

Besides Belgium and Poland, where was Britain willing to go to war simply to preserve another country's territorial integrity?

The guarantees were not a general rule: Belgium mattered to British security, while Poland became the point where Britain and France finally chose to stop German expansion after earlier failures.

The answer argues that Britain opposed hostile control of Belgium because it threatened British security directly. Poland became the line at which Britain and France finally chose to resist Germany after failing to act over Austria and Czechoslovakia. It notes that the same powers did not declare war when the Soviet Union entered Poland, which exposes the limits of a purely legal or moral explanation. The answer instead emphasizes strategic geography, the Polish military's perceived ability to resist, and a succession of missed earlier opportunities.