Surely the Seven Years War in the 18th century was a global affair? There were battles in Europe, North and South America, India and Africa.
You can go back a lot earlier than that - the Second Anglo-Dutch War had fighting (or at least military operations) in Africa and North America as well as the main theatre in the narrow seas.
War of Spanish Succession.
Sometimes in the forum the Trent affair is brought up regarding the possibility of a world wide war. A real world war would mean official alliances and vague modern communications Like the telegraph at least. The Crimean War set some of the trends If modern wars. Maybe 1880es-1890es could have seen a disatrous world war of unseen qualities.'world war' is in quotes here since a truly global world-war was only viable from the late modern age forwards. when was the earliest period a war could happen which involved the vast majority of the world, and the greatest powers? what PoD's could produce this as early as possible?
Fourth Crusade? Crisis of the Third century?
That Portuguese Ottoman war qualifiés.I think a world war needs to be more than just a global war between 2 states.
My minimum points would be:
- Several theatres of conflict across the world
- At least one side an alliance of powers rather than a combatant with outside assistance
The latter saw fighting over a very wide geographical area--from Ctesiphon to Spain, even North Africa saw conflict. There was at least one big naval battle in the eastern Mediterranean. But the third century crisis--its military aspects--was a series of separate wars involving several enemies, not a single big one between two sides.
You can go back a lot earlier than that - the Second Anglo-Dutch War had fighting (or at least military operations) in Africa and North America as well as the main theatre in the narrow seas.