Answer:
The United Kingdom was an Allied Power
Step-by-step explanation:
They joined because Germany (an Axis Power) had invaded Poland, an ally of the UK, who promised to help their allies, just like they had helped Belgium in WWI. Also, the USA was an Allied Power, and the USA helped the UK. Why would an Allied Power help an Axis Power?
Oh, & here's a bonus fun fact: During WWII, there was a Polish tax collector who was a Nazi collaborator. The man had the job of stapling metal tags on the ears of the fattest pigs to indicate that those pigs belonged to the Nazis. One day, though, some Polish rebels gave their village a bit of entertainment by stapling metal tags in his ears. Now, I know he was a Nazi collaborator, but, ah...
...Ouch.
I hope this helps, and have a WONDERFUL day! :D