Believe it or not, at that time Hitler still had the misguided notion that the British Empire and the 3rd Reich could live in peace side by side, ruling the world equally. Hitler had a deep admiration for the British Empire and did not wish to provoke Britain! It was because of this, in the most part, that he made that fatal mistake (or not, depending what side you were on!)
It wasn't long before he snapped out of it though, and started hating the Brits with a venom.
Some historians say that after Churchill had tamed his Cabinet and delivered the "We shall never surrender" speech, then it was only a matter of time before Germany lost the war.
Dunkirk was a huge event in the story of WW2, and it is absolutely right that it should be remembered. My Grandfather was on the beach 60 years ago, I'll never forget!
The RN ships were protecting the merchant convoys crossing the Atlantic and at that time I don't think the UK/US lend/lease agreement had been forged so there weren't that many ships to go around.