It has been hypothesized a lot what would happen if the germans were successful with operation Sea Lion. The theory i tend to agree with most is:
After crumbling to the might of the German Luftwaffe, the royal air force was almost nonexistent. Given the lack of british air cover, the German amphibious and airborne landings launched out of france and norway established a foothold on English soil experiencing the greatest successes in the region of Dover where german air support streamed in almost constantly to harass British Forces there. Once a foothold was established, the Luftwaffe began basing aircraft in southern England to mount sorties in support of advancing Wehrmacht troops. This later allowed for a great number of Panzers to rumble ashore.
As of the time of the invasion, The United States would have declared war on Germany, but this would have had little immediate impact on the situation.
after some time (several weeks if not months) of bitter fighting the decision to retreat would be made, a dunkirk like situation would have occurred. As many military and civilian personnel as possible would have been evacuated, probably from Liverpool, Blackpool and Londonderry aboard any ship available capable of making the voyage to the US and Canada.
A humanitarian and military disaster would have occurred however as BdU alerts atlantic U-boats to gather in grid squares AM 29, 34 and 45 as well as BE 33, 36 and 39 (or thereabout) to interdict the retreating forces. The U-boat fleet would have been in a position to harass the retreating merchant ships and warships for days heading out into the open atlantic perhaps at the cost of many thousands of civilian and military deaths and material losses.
German surface units would have largely been held in reserve to defend against any possibility of naval counter attack and would have continued their policy of avoiding direct action against the RN.
Back in the U.S and Canada, the RAF would re-equip with aircraft largely provided by the U.S. and begin training operations. The ships of the Royal navy that managed to cross the expanse of the Atlantic without falling victim to repeated U-boat attacks would have undergone repair, refit etc at various locations along the United States East Coast.
Iceland as well as Ireland would have then capitulated to Germany without military action. this would allow for u-boat bases about as far out into the Atlantic Ocean as Hitler could have ever hoped for. but with Britain out of the war, the U-boat mission would have to be changed from that of starving the british out of the war to one of attacking the United States from Galveston to Boston and all points in between. While this would have probably been initially devastating to the allies, the result would have likely been the same as Operation Drumbeat. especially with the USN and RN concentrated on the eastern seaboard of the USA cooperatively interdicting U-boats together.
what happens next gets much harder to predict.
For at least a few months, pockets of resistance throughout the UK would be silenced one by one by the German forces there.
Depending on the complexity of the Sea Lion operation and the German losses sustained during that campaign, the situation with Russia may have changed considerably. The German invasion of russia may have been delayed months or years, or may not have happened at all.
Germany would have required quite some time to develop the capability to attack the United States and Canada by air. The FW-200 condor could do it assuming it struck from bases in Iceland, but would have required the use of auxiliary fuel tanks in order to make the trip and as such would have been able to carry almost no armament.
even if they did strike at what few targets would be in their range, their losses would be enormous. The condor was not a super high altitude bomber so most of their flight to the US and Canada would have been in the weather over the north atlantic, and greenland. this would have likely limited them to striking in only a few months out of the summer. Not only that, but the penetration into the United States Airspace would have occurred over a very narrow corridor over a distance of over 1,000 miles of lookout posts, reporting points and possibly even radar sites... P-40s and P-38s of the era would have made any German attack by air a suicide mission.
in the "original timeline" it would be another two years after operation sea lion was abandoned before the "Amerika Bomber" project was initiated. So even with England out of the way, it would be some time before Germany had anything to strike at the US and Canada with other than it's u-boats. It would be like ISIS trying to come up with a drone that could bomb american assets on the moon
a pointless pipe dream with zero chance of realization
The point i guess that im trying to make is that while the U-boats had a tremendous impact on the war and an impact on the way the allies were forced to adapt, the U-boats really had very little impact on the
outcome of the war as a whole - it all came down to a matter of numbers where the U-boats were concerned. It is debated widely how much of an impact the u-boats really had on the war as is. the kriegsmarine only had 26 U-boats to thrust into action in 1939. Donitz himself stated a need for 300 u-boats to starve england out of the war. his logic was that roughly 1/3 of his force would be at sea in combat, 1/3 of the force would be in port for servicing, and 1/3 of the force would be transiting to and from operational areas, ergo he had a goal of 100 boats in combat operations at any given time.
Donitz eventually got his wish, but germany was unable to put anywhere near 100 boats to sea until the summer of 42. this was two years
after the Germans essentially abandoned any idea of invading the UK. as we know, within a year of this point, the U-boats began suffering unsustainable losses, and their combat effectiveness was substantially diminished for the remainder of the war.
and that's where these "alternate history scenarios" fall apart. Overrunning the UK depended on
many tumblers falling into place to unlock the door to german victory. Air, land and Sea assets, all completing a series of operational goals meant to support one another would be required to win it.
The Luftwaffe would have to gain air superiority by defeating the RAF which it was quiet obviously unable to do. the Wehrmacht would have had to employ fallschirmjager troops to take key positions in england behind enemy lines like bridges, airports and radar sites all the while mounting a successful amphibious strike, the kriegsmarine would have to support landing operations using pocket battleships and light cruisers in an area of sea where they were vastly outmatched by the superior Royal Navy. The kriegsmarine would have to operate jointly with the Wehrmacht in launching this endeavor and frankly, lets be honest... the Boy Scouts of America's canoe racing team was probably more prepared to launch a full scale amphibious invasion. this leaves the U-boats, well, they would have had
little to do in
direct support of an amphibious invasion as their main goal from day one was to blockade. In september of 1940... when the invasion of England was supposedly going to take place the kriegsmarine only had about 20 U-boats at sea tasked with covering the vastness of the north atlantic... far short of the 100 boats donitz would need.
... so basing the game's entire campaign on an alternate sci-fi-esque timeline to be influenced by the player's individual ability to achieve goals would be too much of a detraction i would think
They did this with Microsoft Combat Flight Simulator 3 - you go out and fly a sortie and your performance impacts the progress of the front line in that sector. If you do well, that sector advances, if you do poorly that sector retreats. seems like a neat idea, but you could very easily take over the entire UK in a matter of 15 missions while playing as the Germans and it took so much away from any feel of realism or historical accuracy that it made the campaign no fun - at least for me.