Tunisia, a former French colony that achieved independence in 1956, has been the crucible of momentous change since the street vendor, Mohamed Bouazizi, set himself on fire on Dec. 17, 2010, spurring massive demonstrations.

An Islamist party, Ennahda, won the most votes the next year in Tunisia’s first free parliamentary elections, but its proposal for a new constitution that would weaken women’s status as equal to men, among other changes, drew widespread protests. The assassination of two opposition politicians in 2013 led to calls for the Ennahda-led government to step down, and by the end of the year it had agreed to a timetable for a democratic transition, becoming the only Islamist-led government created during the Arab Spring to cede power willingly.

The quartet was successful because it represented a credible third party that could guarantee the agreement. Having seen the Muslim Brotherhood government deposed in Egypt, Tunisia’s Islamists feared a similar fate. Although the Islamists did not trust their political opponents, they reportedly did trust Mr. Abassi, the union leader.