AP Photo

The recent shoot-out in a Gaza mosque between Hamas security officers and militants from the radical jihadist group the Warriors of God brought to the surface the deep tensions that divide Palestinian Islamists. Twenty-two people died, including the Warriors of God's leader, Abdel-Latif Moussa. But Palestinian security officials doubt that these will be the last casualties.

With Hamas in control for more than two years, the Gaza Strip has long been considered much more traditional and conservative than the West Bank. Nevertheless, in Gaza's political milieu, Hamas is a moderate Islamic group that opposes Al-Qaeda-style extremism. But such extremist Islamic groups have been gaining support in Gaza, and Hamas has noticed. The shoot-out in the mosque shows that Hamas will be...