That's WWI, look up the website there; it'll be a long list.
World War One was fought between the Triple Entente or Allied Powers, whcih included Great Britain, France, Russia, Italy (As of 1915) and the United States (As of 1917), along with other nations, and the Central Powers, which included Germany, Austria-Hungary, the Ottoman Empire, and Bulgaria (As of 1915). World War 1 was mainly fought in Europe, though fighting also took place in Africa, Asia, and the Pacific.
Many American nations fought wars of independence.
Spain and the US.
Argentina and Great Britain
UN-United Nations
About 50 nations fought in WW2 but basically the whole world was involved.
Argentina and The United Kingdom.
Iran
buttholes
The northern front was a unit of the Russian army that was formed in August 1915. The unit fought on the Eastern Front during World War I.
A civil war is a war fought between the inhabitants of one country, for example, the US Civil War was fought between the South and North US. However, a "normal" war is fought between 2 opposing powers.
The United States fought against the United Nations during World War I and then fought Canada and Emperor Palpatine's droid army after nuking the UN headquarters in New York. The US won.