A fateful battle between secularism and Islamism is unfurling at Europe’s gates.

On Tuesday, the highest court in Turkey blocked a Muslim fundamentalist candidate from running for president. At least 700,000 people had marched to protest the nomination in Istanbul on Sunday, and the Turkish army’s chiefs of staff have warned the elected Islamist government of Prime Minister Erdogan to desist from diluting the secular nature of the country, or else.

It is not an idle threat — the army has removed four governments since 1960.

At issue is the candidacy of Abdullah Gül, a close ally of Mr. Erdogan’s and a man of strong Muslim fundamentalist tendencies. The ruling Islamists have vowed to reintroduce Mr. Gül as candidate; even if they do not, their backing will go to someone from the emerging class of religious Turks Mr. Gül represents.

Presidential power in Turkey is far from symbolic. According to the Turkish constitution, a president must approve every law and formal decision emerging from the parliament, as well as the appointments of governors, police chiefs, ministry department heads and their deputies, and members of Higher Education Board, university rectors, and the head of the central bank.

Under Turkey’s parliamentary system, an Islamist president teaming up with an Islamist prime minister and parliament would create a cabal of irreversible power that could continue changing the face of secular Turkey.

As it stands, the Erdogan administration’s current program is “Arabizing” Turkey as well as Islamizing it.

Some of the largest inflows of foreign investment in the past three years have come from the current prime minister the United Arab Emirates and emir of Dubai, Sheik Mohammed Al Maktoum, whose country is an open front for Iran.

Billions of dollars have also come from King Abdullah of Saudi Arabia and a Saudi mega-businessman, Prince Al-Waleed Bin Talal, as well as from the ruling families of Kuwait and Qatar. The resulting change is palpable on the streets of Istanbul, Ankara, and other cities.

“The face of Turkey is changing; I look at people in Ankara and see more and more people in Saudi garb,” a former chief of the general staff, retired General Dogan Gures, said to journalists on Haberturk TV. “Verbal expressions are changing to Arabic expressions. There is nervousness that Turkish secularism is in danger.”

At stake is the modern secular republic established in Turkey by the nationalist leader Kemal Ataturk in 1923. Ataturk, an army man, made huge reforms to what had been the center of the Ottoman Empire, among them giving women the right to vote, restricting Islamic dress, and replacing Arabic script with the Roman alphabet. His rule created new paradigms for modern Islam and were a sea change in the heart of the Muslim Middle East.

Obviously, the Ataturk revolution has not been secured. And the threats to it come at a time when political Islam is invading Europe next door, establishing outposts among Pakistani minorities of Britain and the Arab immigrant ghettos of France, Belgium, Spain, and the Netherlands. In all these places, Islamists have staged successful cultural intimidations: forcing the veil on their women and obtaining Shariah law exceptions for themselves, as well as fomenting terrorist bombings and killings in Madrid, London, Paris, and elsewhere.

For Europe, the loss of a secular Turkey is impossible to contemplate. Turkey is an industrious nation of 71 million Sunni Muslims, a significant military power, a NATO member, and a key American ally. Aside from Israel, it has been the only functioning Middle East democracy, and it has borders with Iran, Iraq, and Syria, so its turmoil cannot be ignored.

Because of its significance in the region, tending to Turkey is far more important than even winning the war in Iraq. If the Islamist barbarians can triumph in Turkey, their next stop would be America.

So as hard as it is to conceive and to admit, a coup may be the only way to go.

Even if a new election were to be held in Turkey, under the present structures, the Islamists are still likely to dominate. When Muslim fundamentalists hijack a democracy, radical solutions may be the only choice.