Reasons for declaring war in 1939?

Answer 1

Germany declared war on Poland after the "Gleiwitz incident". The Gleiwitz incident was when some Polish troops crossed the border into Nazi Germany, attacked a radio station in the town of Gleiwitz, and sent out an anti-German message over the radio. However, it was a ruse. They were not actually Polish troops, they were Nazi agents pretending to be Polish- an action popularly called a "false flag" operation.

Adolf Hitler had previously made it clear that he wanted to take land from western Poland that had belonged to Germany until World War I. When Britain and France refused to allow it (as they had with Czechoslovakia), he had to come up with a reason to attack Poland. The Gleiwitz incident, and several other smaller false flag operations that mostly involved attacks on railways and government buildings, were given as justification. Of course, nobody really believed the Polish actually did these things.

France and Britain had promised to come to Poland's assistance in the event of a war with Germany. After Germany launched their invasion of Poland, France and Britain did indeed declare war on Germany- although they famously did little to actually help Poland.

Answer 2

Britain declared war because Germany invaded Poland, with whom Britain had a Treaty.