Why? Germany did not declare war on Great Britain. Check it out.
You are right Samsette, Neville Chamberlain Declared that Great Britain was at war with Germany and NZ, Australia, and Canada did likewise within hours.
The Germans accepted the situation.
Franklin D Roosevelt effectively brought the USA into the conflict on the Allies side well before Pearl Harbour forced him to openly declare and he did this by changing a law that enabled the USA to manufacture all forms of armaments for supply to Allied countries.