Nearly all of the world's countries participated in World War II.
Notes on verification
Corroborated by Wikipedia's dedicated 'World War II by country' article and an independent academic source; consistent with historical estimates that most of the era's ~70 sovereign states were involved as belligerents, occupied territories, or resource contributors.
Sources
- World War II (en.wikipedia.org)
- https://graphsearch.epfl.ch/en/concept/32927 (graphsearch.epfl.ch)
- https://veteransbreakfastclub.org/the-four-essential-facts-about-world-war-ii/ (veteransbreakfastclub.org)