Nearly all of the world's countries participated in World War II.
Notes on verification
Consistently corroborated by Wikipedia and independent mirrors/aggregators; qualitative claim ('nearly all'/'vast majority') is well-supported though precise country counts vary by source.
Sources
- World War II (en.wikipedia.org)
- https://graphsearch.epfl.ch/en/concept/32927 (graphsearch.epfl.ch)
- https://military-history.fandom.com/wiki/World_War_II (military-history.fandom.com)