The most significant obstacle to any maritime invasion of the British Isles has always been the surface fleet of the Royal Navy. From the mid-19th century when steamships became the predominant form of naval vessel, British defence planning became increasingly worried about the 'Channel Dash', the idea that sufficiently fast steamships would be able to cross the Strait of Dover and land ground forces before the bulk of the Home Fleet would be able to respond.

With Operation Sealion being planned in 1940, the Royal Navy home fleet was still the most significant problem. Invasion barges would only be able to safely cross if the Royal Navy could be kept at bay, and that in turn could only be achieved by achieving air superiority over the RAF to allow the Luftwaffe to devote its attention to bombing ships and harbours unimpeded. This could only realistically be achieved on the South/East coasts of England; the Messerschmitt Bf109, mainstay fighter of the Luftwaffe in 1940, had a range of some 530 miles which meant that while it could theoretically reach Britain and return from Scandinavia, it would have virtually no capacity for manoeuvre or combat over its target. If British land-based fighter cover over the North Sea could continue unimpeded - and with no German equivalent of the Fleet Air Arm to provide air support to the Kriegsmarine, the Royal Navy would have been readily able to defeat any invasion attempt across the North Sea.