Why did Britain go to war over Poland in September 1939?

Soon after, Hitler announced his plans to annex Sudetenland. British Prime Minister Neville Chamberlain and French Prime Minister Édouard Daladier had both met with Adolf Hitler and Benito Mussolini in the Munich Conference. Chamberlain appeased Hitler by allowing him to take the Sudetenland. This, however, would leave Czechoslovakia without a defense, as the Sudetenland was a chain of mountains that provided a natural defense to the country. Hitler announced that he wanted to take Sudetenland to protect fellow Germans (around 3 million Germans were present in Sudetenland). Soon after the conference, the German army invaded Czechoslovakia and took it.

Italy had invaded Ethiopia and Albania, yet the League of Nations took no action. Ethiopia called for military protection, yet the league thought that it wasn't enough to create war

Britain and France could no longer ignore the aggressors and had begun to mobilize for an impending war. They vowed to protect Poland if it had come under attack, and it did. Germany believed that Britain would not honor its agreement to defend Poland like with Czechoslovakia, but this proved to be wrong; Britain issued Germany an ultimatum and they ignored it. The war had started at the invasion of Poland. People spoke of the "phony war," a war over Poland after Britain and France declared war on the Rome-Berlin Axis. It was called this because Britain and France had lacked major military operations, and people hoped that a larger war could be averted. Suffice to say, it was not, and World War II had been set up.

There was no military intervention by Britain during the September Campaign either within or beyond Poland nor within Poland any time later. Chamberlain thought that it was not worth the risk nor the cost for Britain to step in and try to stop a fight between Poland, one of its allies, and Germany, one of its enemies, when Poland was invaded by Germany on September 1, 1939, it was conquered in less than a month, and was then quickly parceled out between Germany, one of Britain's enemies, and the Soviet Union, one of Britain's allies. Chamberlain declared war on Germany after the invasion of Poland, but no troops were actually sent to help defend Poland. There was a period of a few months called the 'phony war' where the two countries were technically at war.

Britain began to fight Germany only after a continental threat emerged and Britain realized that they will be affected as well unless they'll fight.

Britain went to war over Poland.

In this essay I have explained why Britain went to war over Poland.