As far as I know, there are 10 playable countries. The UK, France and the US have the same type of units (e.g. their interceptor is always the Grumman F4F Wildcat). Same with Italy, Spain and Germany, and Russia and Turkey also have the same. Sweden is also German. What I suggest is the UK not using the same units as the US (they used their won tanks too), and France would also be independent (they used Renault tanks). Italy also used their own tanks. Spain could depend on Germany, and Sweden could still depend on the US. Turkey should use British tanks.
The past is a foreign country.