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By many measure, Tunisia has the best chance among the Arab Spring countries to transition to democracy. It is a moderate Islamist-led state with close ties to the West. Nearly two years after deposing one of the region’s most repressive autocrats, Zine el-Abidine Ben Ali, Tunisians are deep into the crucial task of writing a postrevolutionary Constitution.

But a spate of recent violent incidents, including attacks on the American Embassy in Tunis last month, have fueled new tensions between the moderate Islamic government and liberal secularist opposition parties over Islam’s role and the best way to handle extremists. How those tensions are resolved will determine Tunisia’s future as well as the broader regional debate over whether Islam and...