Upvote:0
The Chinese Civil War comes to mind, since there two chinese factions (Nationalists and Communists) and one foreign faction, i.e. the Japanese. The alliance between Nationalists and Communists wasn't a very close one.
However, I am not quite sure whether this conflict matches the battlefield 1 & battlefield 2 requirement.
Upvote:2
A possible example would be the Battle for Castle Itter, a World War II battle which took place on the 5th May 1945, in which American and German troops fought side by side against other German troops.
Upvote:3
On the 9th October 2019 Turkey started Operation Peace Spring, invading north-eastern Syria. In response the Syrian Democratic Forces (SDF) joined forces with the Syrian Arab Army (SAA) to respond to the invasion. The SDF (as well as other groups) and the SAA have been engaged over much of the country since the 2011 uprising, and are currently engaged in Idlib.