Can anyone think of a way from 1900-1950 there could have ever been a worldwide conflict with military engagements on all 6 inhabited continents i.e a "true" world war?
Really, we pretty much have that. In OTL's WWII...So now you just need to find a way to bring the War to South America, and voila.
Doesn't the Battle of the Rio Platte count? (Or was that World War I?)
Unless I am wrong, WWII counts:
Europe: Many Battles
Asia: Many Battles
Africa: Many Battles
North America: The Aleutian Campaign
South America: The Battle of Rio Platte
Australia: The Darwin Raids
FWIW, although outside of the time frame, The Seven Years War also had fighting on all known continents; Australia and Antarctica not yet known to Europeans.
Do you mean ground battles? Because that would just exclude South America and maybe Australia (unless you count certain Pacific islands as being part of Oceania), but the Japanese straight-up invaded Alaska which is undoubtedly a part of North America and there were naval battles on the South American coast as well as bombings in Australia.I think they want many battles on all six inhabited continents.
This scenario is set to happen in Jello_Biafra's Reds! TL. An Integralist Brazil signs a defense treaty w/ the Nazis and forms a New World Axis w/ Venezuela and Peru, all in response to the southward spread of socialism from the UASR. Not sure if there'll be any combat in Australia in that TL.