GERMANS
The enemy.
The question is unanswerable as asked since it does not define who the "enemy" is.
General Patton did not believe that the United States fought the wrong enemy during World War II. He was a strong advocate for defeating Nazi Germany and believed in the importance of fighting against the Axis powers.
The code that was never broken by the enemy was based on the language of Navajo. Neither the Japanese, nor the German could break the code words. The code was used by people that fought in the Pacific war during world war 2.
No comparision. The Korean War was fought against a determined organized enemy.
World war three has not been fought yet.
World War 1 was fought against primarily Germany.
World War I was fought between 1914 and 1918.
Because the US and USSR shared a common enemy. Germany invaded the USSR in June, 1941, and in December, 1941 Germany declared war on the US. My enemy's enemy is my friend.
The Roman empire never faced a 'world war' - not even one within the confines of the Empire. They fought individual wars, one at a time and never faced any one enemy in more than one 'theatre of war'.
"I know not with what weapons World War III will be fought, but World War IV will be fought with sticks and stones." ~ Albert Einstein
The United States of America fought the Confederate States of America in the Civil war.