The countries that fought in WWII were divided along the lines of the "Axis" and the "Allies." The Axis powers were dominated by Germany in Europe and Japan in Asia but they included their allies of Italy, eventually the French, at the beginning Russia, Austria, etc.
The Allies consisted mainly of the British and the United States but then grew to include Russia after Germany chose to invade Russia and break their alliance.
Of course other countries were involved, the exiled government of the French, the Canadians and others fought on the side of the Allies. And fighting took place in a number of countries that really had no allegiance to either side but were forced to pick one or the other.