Looking for a book...two guys (mine inspectors?) are trapped underground during cave-in or earthquake, and find an underground civilization. This consisted of two warring factions. The two guys are separated, and unknown to each other both become leaders of the two factions (because of their physical superiority). They try to stop the ongoing war, but are discovered and flee back to the surface, resolved to return some day and stop the conflict.
The book dates from the 60s or so. Any clue?