Neville Chamberlain "On Hitler's Invasion of Poland"-- Why does Chamberlain believe a declaration of war against Germany is necessary ?

The fundamental premise of Chamberlain's address to the House of Commons in speaking "On Hitler's Invasion of Poland" is that war between the United Kingdom and Germany was to be avoided.  Chamberlain makes the painstaking case that he pursued open and back door diplomatic channels through Germany to ensure that Poland will be left untouched.  The policy of appeasement, to which there is mild reference, was vociferously pursued by the European leaders, especially Chamberlain.  This policy stated that if Hitler and the Nazis were given lands in Eastern Europe, they would promise through written agreement to leave Poland alone.  Unknown to Chamberlain in his address is that Hitler and Russia's leader, Stalin, had agreed to carve up Poland between them, and Russia would be left alone.  This guaranteed that Hitler would not have had to deal with Russian threats, and would move close to his goal of complete control over Europe.  It is evident in this speech that Chamberlain had no idea that this agreement was reached.  We hear consistently of the idea of "this white piece of paper," referring to the original agreement Hitler signed indicating he would not attack Poland.  When Chamberlain makes his case for war, he does so with a noticeably heavy heart indicating that "Everything that I have worked for, everything that I have hoped for, everything that I believed in during my public life, has crashed into ruins."  He is resigned to the fact that war is needed, and his speech makes clear that he wants to expand the military draft for British men, expand the ability to use the armed forces for defense and enter into the conflict.  His declaration of war against Germany is necessary because he feels that Germany's motives are clearly aggressive and confrontational and that the German nation, motivated by "Hiterlism," is not going to honor any diplomatic agreements.  Chamberlain's original belief of "peace in our lifetime," is permanently dashed when Germany invades Poland.  In the process, Chamberlain also realizes that his politics of appeasement, something that he convinced the British public to adopt, is forever repudiated and rejected.  It would only be a matter of time before he would be repudiated and rejected and Winston Churchill, complete with his Bowler Hat and Cigar, would assume control.