On Feb. 6, 2013, Tunisia entered into unknown territory. The revolution seemed to be devouring its members in an unprecedented wave of bloodshed. The killing of activist Chokri Belaid in stark daylight warned of a potential series of political assassinations, which would be exacerbated by the country's security situation.

But the most terrifying part was the involvement of the ruling Islamist party — the supposed product of the struggle against the regime of former...