It's not quite as simple as protecting England [UK]. The American public were naturally reluctant to become emrboiled in yet another European war. However, the Americans were involved from the very outset in that the US Navy were busy escorting British convoys from the US Eastern Seaboard as far East as Iceland where the Royal Navy took over the job of convoy escort. That was back in 1940 - well before Pearl Harbor or any thought of the USA joining in a war in Europe.
From September 1939 through until January 1942 the British having been forced out of France by the Germans, then stood alone. Britain and her allies fought what became known as the Battle of Britain.
Now let me introduce you to the Eagle Squadron Royal Air Force - all the pilots were Americans.
http://www.youtube.com/watch?v=_z2fVNnI6ms
Don't let people fob you off with the idea that the Americans did nothing to help, they did. It's just that their government back in Washington was unable to act openly and instead had to do everything in a secret manner to aid Britain.
The Americans came up with an idea that became known as Lend Lease. No one has ever satsifactorily explained that to me, because my dad tole me that he had lost his weapon somewhere in France during the retreat to Dunkirk and he reckoned that it must have been paid for by the Americans. It's probably still out there somewhere, in a ditch, slowly rusting away. Who knows?
Following the dastardly attack upon Pearl Harbor by the Imperial Japanese Navy - a date which will live in infamy, the American people and their government turned their attention then to a war against Japan.
Adolf Hitler, no doubt in a flash if inspirational madness, declared war on the USA. Quite naturally this annoyed the Americans, who in turn declared war upon Germany.
Blah!
The rest is history.
But like I say, the Americans were swift to aid the UK but had to do it in a clandestine manner from the beginning but from about January 1942 the United Stars Army GIs marched down the gangplanks of their troops ships at Liverpool - and etc you know the rest.
Re the Enigma machine. The first people to grab hold of a version of this cypher wonder thing, were the Poles who got hold of on in c1938 before the outbreak of WW2. It had three cypher wheels. Unfortunately for the Poles and for some obscure reason, the Germans decided to add a 4th wheel before their dastardly and vile attack upon Poland.
The strong and often quite friendly relationship between the US and UK, goes back a very long way. But there was a time when we did this. . . .sorry
http://www.youtube.com/watch?v=SJMbxZ1k9NQ