There are two different reports, the most common is that Germens struck first. But there are two small flaws in this.
First, Hitler wanted peace with England. He thought the English and Americans would back him up and stand with him on the top of the new world order. Seeing that we didnt like what he was doing, that didnt go over well.
The seconded is the the US and England (supposedly) that the US and England where tyring to push Germeny into open war with them by "poking" them. The US had Uboats out and England had bombs. If this is true, then England his first.