Bede

I've been playing as the Allies in the whole campaign (starting in 1939). Eventually it comes time for the Americans to invade Tunisia/Italy/Spain, etc. No matter what I do I can never get anywhere. My units don't have enough supply to do anything and they can't dislodge the Axis units defending in cities. Today I tried to invade Portugal (Spain had gone Axis and the Germans had taken Portugal). I landed units to take the city in Portugal and supported it with naval bombardment. The naval bombardment didn't reduce the entrenchment level for some reason and my multiple units couldn't reduce the defending German unit. Please give me advice on how to to an amphibious invasion properly. Thanks.