What were the factors that led to Chamberlain's policy of appeasement changing from 1938 to 1939?

The events between 1938 and 1939 caused Chamberlain's policy of appeasement to change. Since October 1938, it was obvious that Hitler's next target was to conquer Poland. Chamberlain now realized that his appeasement policy wasn't going to help any longer. This is was the end of the appeasement policy. Events in Europe pressured Chamberlain to abandon the appeasement policy. At this point Britain's tasks to protect its imperial colonies were becoming harder. Since December 1938, Britain began to prepare …

…triple entente is signed. After Hitler invades Poland on September 1st, Britain declares war on Germany. Although Chamberlain was trusting Hitler too much, he was already preparing for a war, because he now realized that appeasing Hitler was now becoming useless because Hitler was becoming unstoppable . After a series of events which took place in 1938, Britain was already increasing its army. After the invasion of Poland follows the declare of war by Britain to Germany.