As for Potsdam it seems the Allies wanted to make deals for postwar Europe without dealing with France's interests. Much of this has to do with Stalin and Roosevelt. Stalin felt France should be treated much as Poland; and Roosevelt did not feel need to deal with them. Churchill thought otherwise and realized they would be a strong future ally.

A number of plans for POtsdam clashed with DeGaulles ideas and that is probably why he was not invited. For one thing how Germany would be regulated, the French were opposed to a centralized government and the allies favored that. Also, the allies intended to divide indochina into northern and southern spheres of influence, one for China and one for UK. This led to practical difficulties owing to the weakness of China. Roosevelt favored this, but then he gave in and would allow France to govern it as a trustee of sorts. Also DeGaulle had been pushing to occupy the left back of the Rhine, and getting france to protect the border along the Rhine. Lots of squablling between US and French forces in southern Germany as the de Gaulle ordered troops to occupy as much of Germany as possible.

See Charles deGaulle's Legacy of Ideas, edit. Ben Rowland, p 40-41.