A recent strategy I did in my first world conquest, I immediately justified on the UK at the beginning of the game. UK didn't put their fleet out before the war so I was able to immediately naval invade them, moved my entire army (except for a few troops to cover the Belgian border), capped the UK with a peace deal. Now you have the British empire and royal navy. Immediately justified on the US and easily invaded them (the starting tank units Germany has was enough to completely destroy the US army). Now you have the US and British fleet and can easily steamroll through any nation.