Spain.
Platt Amendment Foraker Act
Spain.
Spain and the US.
Spain was the other belligerent.
USSR aka the sovet union
During the US Civil War, Confederate General Robert E. Lee was confronted by a number of Union commanders in the Eastern theater. He ranked his three most able opponents as US Grant, George B. McClellan and George G. Meade.
Generals Lee and Grant were each in the Mexican War as US officers. Later they both were in the US Civil War as opponents.
Germany, Japan, Italy for the most part
The US and the Soviet Union were opponents during the Cold War period of 1945-1990, because they had different ideologies and objectives for the world. The US wanted a world based on the free market, democracy, and independent nations, while the USSR was trying to bring about a world of a command economy, dictatorship, and control of all nations by the USSR.
{| |- | The primary opponents were the US and the Soviet Union. While they were opponents, they did not actually have a war going on that was hot, they weren't shooting at each other. There were numerous engagements with others that were bullets were flying. |}
US/Allies verses North Vietnamese Army/Air Force/Navy and VC.