It is important for the United States to tackle radical Islamist ideological indoctrination — dawa — before it takes root to the extent it has in Europe.

Turkey's President Recep Tayyip Erdogan recently escalated his feud with the German and Dutch governments, accusing them of being Nazis. At issue is Mr. Erdogan's desire to have his government campaign among the Turkish diaspora in favor of a referendum that would give him nearly unlimited powers. The Dutch and German governments have so far denied the Turkish government's requests to campaign inside their borders.

More problematic, though, is the influence that Mr. Erdogan and his government wield among Turks living abroad through Diyanet, Turkey's official religious affairs institution. Diyanet has sent hundreds of clerics to preach at mosques throughout Germany, although many do not speak German. The aim of Diyanet preachers is not assimilation into Germany's civic institutions and values, but a type of cultural and religious separatism for Muslims of Turkish descent. As Mr. Erdogan increasingly implements political Islam in Turkey, his government's spiritual influence in Europe is a cause for concern.

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