Islamists are or very soon will be in control of Turkey, Iraq, Saudi Arabia, Gaza, Egypt, Libya, and the Sixth Caliphate of Tunisia. Syria, which always effectively means Lebanon as well, is next on the list. And then, saving perhaps any extensions all the way to the Atlantic, the Crescent will be complete.
This is usually presented as a threat to Israel. But in fact the Saudi-backed side in Lebanon is the side favoured by Israel, relations between Israel and Turkey were more than cordial until Israel fired on Turkish civilians on the high seas (how touchy of the Turks - Israel long ago attacked a United States Navy vessel, with much loss of life, to absolutely no adverse reaction from the Americans), and Israel's vast nuclear arsenal and other military capabilities, all entirely at American expense, make her the untouchable regional superpower.
In any case, the fiefdom of the Islamic Movement in Israel, Umm al-Fahm, recently voted by a large margin to remain in Israel rather than be transferred to Palestinian jurisdiction, while its former Mayor and the Islamic Movement's leader, Raed Saleh, cheerfully travels on a passport of the country with its name in that of his Movement, including when he is being thrown out of Britain. Everyone born into certain ethnic minorities in Israel is automatically subject to Sharia courts whose judges are appointed and paid by the State. At least one member of the Knesset, Ibrahim Sarsur, has called for Jerusalem to become the capital of the Caliphate. Sarsur's parliamentary oath obviously does not preclude such a position.
No member of a Parliament sitting at London, Paris or The Hague has yet made that suggestion in respect of any of those cities. Leaving aside Syria's Christians, Sh'ites, Alawites and Druze, the danger from the completion of the Crescent would not be to Israel. It would be to us, as the completion of the Crescent became the completion of the Full Moon.