Some countries have accurace (or as accurate as possible) records of who fought in their forces in World War I; but some countries for various reasons have little or no records of this information.
50 million from all 57 countries involved in the conflict
No British counties fought against each other in ww1 England, Wales And Scotland All Fought Against The Germans/Austro-Hungarians In The First World War.
World War I was fought in many countries. The "most famous" battlegrounds are in France and Belgium, but fighting also occurred in Serbia, Russia, Africa, the Middle East, Asia, Pacific islands, etc. It was a world war, after all.
none of the world wars were litterally a world war, not all of the countries fought on both wars and the countries didnt fight everyone on each other in both wars, WW2 was classified as a war world because it splitted most of it [Europe and USA joined it too] into 2 groups that fought each other: Allies and Axis.
Japan had a common cause. Austria, Hungary, Italy ,Rumania, Bulgaria & Finland all fought against Russia.
It was the nazi war fought in Poland, Germany, all those countries
All were fought about the whole lenth of the war
Most of World War 2 (but of course not all) was fought in Europe.
Then end of World War I in November 1918 was not completely due to American involvement. The allied effort consisted of many countries and every one contributed to the surrender. They all fought and many from all of the allied countries died.
Countries that won the war were: France, the UK, Italy and the US (and all the other countries accept Russia that fought for the Entente Powers). They wanted the world to develop the Europian way, which we can now watch on the EU countries
The countries involved had colonies all over the world, and they were at war too.