When Poland was invaded, Britain was honour bound to declare war on Germany.
This might not make sense to some, and the situation might seem to have been engineered to cause a war, and then each acident or incident used as an excuse to escalate the violence.
Appeasement of Germany was - in my opinion, an error. Germany was going bad internally long before it started the liebensraum (spelling?) expansion.
As has already been pointed out, the German government thought that Britain would not oppose them. We certainly held many of the same attitudes, anti semitism was only one - but Germany was detaining its intellectuals, and running on strong but strange idealistic lines for years.
I have often wondered how WWII was put off for so long, why Germany had to push so hard and so long to get the full attention of the rest of Europe.
All I can think of is that American isolationism allowed Germany to believe that it could win a European war. Enough Europeans agreed with them, so their governments were putting off the inevetable believing that Germany would dominate Europe from the second half of the 20th century onwards.