It was the Cold War,which started in 1945,and ended in 1991,but there are still tensions between the two countries today.
The cold war
The Cold War.
The US and Mexico.
the two sides during the US civil war were the union (North) and the confederacy (south)
Once Germany surrendered, it was the US against Japan.
US, UK, France, Russia ,Italy v. Germany, the Austro-Hungarian Empire, the Ottoman Empire, and Bulgaria.
In which conflict? They have been on the same side in many wars, on completely opposing sides in others, and neutral (even through their allies) in others.
They were on opposing sides, but they did not fight any organized battles. The US went to the Western Front and the Austrians were on the Eastern and Italian Fronts.
The most US soldiers died during the American Civil War. That is because Americans were fighting on both sides.
'Russia' was too busy to involve US into a World War I as there was a revolution in Russia at that time.
In World War II, the two sides were the Axis, headed by Germany, Japan and Italy, and the Allies, headed by the UK, Russia and the US.
I believe the question might be referring to Russia, who "switched sides" midway so to speak.