The official declaration of war against Germany (on September 3, 1939) came about 15 minutes after the deadline for the final ultimatum to Hitler, as Chamberlain said in this broadcast to the British nation.

Something tells us they weren't expecting him to cave.

Chamberlain's speech had three primary aims:

1. To explain that Britain is at war
2. To explain why
3. To justify his prior appeasement policy

The war declaration was pretty straightforward: The Germans had had until 11:00 that morning to stop their invasion of Poland or else.

They chose else.

Chamberlain explained: "I have to tell you now that no such undertaking has been received, and that consequently this country is at war with Germany" (source).

He described how Hitler was using propaganda to lie and justify the invasion of Poland, and what the German dictator's actions had proven: "His action shows convincingly that there is no chance of expecting that this man will ever give up his practice of using force to gain his will. He can only be stopped by force" (source). Admittedly, this seems obvious to us now, but hindsight is 20/20 and we don't have the trauma of living through World War I to affect our opinions of getting into the sequel.

Chamberlain remained insistent that the government's attempts at appeasement were the right course of action. He said, "I cannot believe that there is anything more or anything different that I could have done and that would have been more successful. Up to the very last it would have been quite possible to have arranged a peaceful and honorable settlement between Germany and Poland, but Hitler would not have it" (source). He wasn't wrong, technically, but Winston Churchill probably had a very different opinion.

Chamberlain ended the speech with a sort of early call-to-arms, encouraging the British people to prepare to get involved in the war effort in any way they could. Like King George VI or Churchill himself, Chamberlain was trying to explain the situation leading to war and prepare Britain emotionally to face the tough road ahead. Chamberlain, being a much more reserved man than Churchill, kept the emotion out of his words, but the intent was clear.

You can hear the speech here.