First movie begins before ww2 with the rising of the third reicht.
The whole first movie is spent on ww2 but a alternate ww2. In this ww2 hitler doesnt make all the stupid misstakes and defetats both russia and uk, japan doesnt drag usa into war(and therefor usa never develops a nuke in that time period) and germany takes over all of europe and japan conquers asia.
I dont know what part australia, south america and africa would play, maby they would have some part in the later movies.
second move, a new cold war but this time with 3 major players, Japan, Germany and USA. The alliance betwen Japan and Germany is destroyed by some event(maby usa stages something that makes germany think japan has betrayed them) and a full scale war betwen those 2 begins at the same time as the german, japan and us navy battles all over the oceans in the world. The movie ends with one of the super powers testing a first nuke.
Third movie this I dont know what it should be like, it could be a feary tale and usa defeats all. Or maby japan and germany once again becomes allies(if usa was the one that developed the nuke)and invades usa. In this movie the 3 powers are totaly in ruins, cities destroyed, armys tired and weak. A very dark and depressing movie in every sence and a dark depressing ending would be most suitable.
The movie have several parts where you follow the soldiers in battle both in air, on the sea and on land. But also alot of story building parts following the leaders of the nations and going semi deep into the politics and resource management of the whole thing. The whole triology would take place over like 20 years or atleast 15. That would make it perfect to follow patriotic rookies in the first movie that in the third movie have become battle scared veterans that doesnt care for anything anymore.