The time must have come to consider whether it is really acceptable to retain Turkey as a member of NATO. At various times in the history of its membership, dating back to 1952, Turkey, though effectively rescued from threats from Stalin by the Truman administration in 1947 and 1948, was a double agent between the Soviet Union and the United States, taking substantial aid from both.

For decades, on the strength of that NATO membership, Turkey knocked noisily on the door of Europe but was generally rebuffed as a nation of Muslims unassimilable to the pretensions of the surging Euro-federal ideal. This remained true in the brief shining but somewhat infamous moment when most of the West European leaders thought that, with the collapse of the Soviet Union, the United States could be eased out of Europe, and the Germans, French, British, Italians, Spanish, and others could stand on one another’s shoulders and Europe would become the center of the world again, after the aberrant century that started with World War I in 1914.

In these circumstances, NATO eroded, first into the enfeebled “coalition of the willing,” which isn’t an alliance at all, just an assertion that if one country in the group wishes to do something, another, if it is in its interest too, might join in. And in these temporarily relaxed times, when European officialdom was aflame with the anticipatory joy of being the world’s greatest power again, it was an affordable luxury to brush off the heirs to the “Abominable Porte,” the “sick man of Europe”; and to do otherwise, as Gladstone said of Disraeli, would be “backing the wrong horse.”

Turkey was one of the world’s greatest powers from the rise of the nation-state in the 16th century, when Europe’s greatest leaders were the Holy Roman (including the Spanish) Empire’s Charles V, Britain’s Henry VIII, France’s Francis I, and Turkey’s Suleiman the Magnificent. The Habsburgs and Romanovs beat the Turks back in the Balkans, Ukraine, and Caucasus through the 18th century, though the Turks besieged Vienna in 1529 and 1683 and almost gained control of the Mediterranean at Lepanto in 1571.

The Ottoman Empire still put up a tremendous fight in World War I, which it made a serious mistake entering, but it sent the British Empire and the French packing in Gallipoli in 1915, inflicting 250,000 casualties and almost ending the career of Winston Churchill. The subsequent regime of Kemal Ataturk and his heirs westernized the alphabet, attire, and customs of the Turks and secularized their government. The army became the custodian of secular democracy, which necessitated overthrowing the democratically elected government from time to time, when it was deemed either insufficiently democratic, too corrupt, or a threat to the continuity of the army’s role as supreme arbiter of the state.

Under Ataturk’s successors, Turkey behaved quite responsibly in the world but did not match its enthusiasm to join the West with very visible economic or social progress. The war with the Kurdish nationalists in Anatolia was prosecuted rather brutally, and Turkey has never come close to West European standards of individual liberties or what became known as transparent government.

Where it all began to become very complicated was with the elevation as prime minister of Recep Tayyip Erdogan in 2003. Mr. Erdogan, a former soccer player who was elected mayor of Istanbul in 1994, formed a modestly Islamic party with the eminent moderate Muslim theologian Fethullah Gulen, who approves of secularism as long as it isn’t rampant materialism, supports good relations with other faiths including Judaism and Christianity, and has a relatively enlightened view of the status of women.

Mr. Gulen departed for the United States in 1999 for specialized medical care but was shortly after accused of calling for a more Islamic government than the Turkish constitution permitted. He claimed to have been taken out of context but has remained since in the United States, at Saylorsburg, Pa., where he continues to exercise great influence in Turkish politics. Mr. Erdogan was removed as mayor of Istanbul, imprisoned for ten months, and banned from public life for publicly reciting a Muslim poem that the chiefs of the army considered subversive.

The plodding and corrupt course of the Turkish government enabled Mr. Erdogan, with Mr. Gulen’s support, to win the 2002 elections, although Abdullah Gul, one of their supporters, was prime minister until the ban on Mr. Erdogan was lifted. Mr. Erdogan was prime minister from 2003 to 2014, cleaned house effectively, purged the army of its more political senior officers, negotiated a partial settlement with the semi-terrorist Kurdish independence movement, known as the PKK, ramped up economic growth, and pursued membership in the European Union.

Generally the West was a little uneasy with an avowedly Islamist government in Ankara, and after the first five years of Mr. Erdogan’s government, matters began to deteriorate; Mr. Erdogan reinterpreted foreign policy as being a protector of Islam and the Arabs and railing vituperatively at Israel, sponsoring violations of the arms ban on Gaza, and threatening the right of Greece and Cyprus to extract offshore oil and natural gas.

In a syndrome that usually afflicts authoritarian governments, Mr. Erdogan became steadily more heavy-handed and corrupt, while trying to distract his electors with shrill, traditionalistic, Israelophobic, and anti-Western rhetoric. His former ally, Mr. Gulen, from Pennsylvania, joined the opposition, as scandals involving many billions of dollars mushroomed up and Mr. Erdogan was caught on tape uttering self-incriminatory reflections on the misuse of public money.

This led to repressive measures against Internet comment on political matters, and Mr. Erdogan claimed that the resulting indictments against some of his officials were an “attempted coup” by Mr. Gulen. Mr. Erdogan had to put down widespread anti-government rioting and had to shelve his plan radically to strengthen the powers of the presidency, but was elected president earlier this year with 52% of the vote in a heavily contested election.

In the Iraq War of 2003, Turkey at first said the Americans and their allies could attack Iraq from eastern Turkey but then reneged; there was no thought of Turkey’s joining the attack, only permitting that it be launched from Turkey. In the recent disintegration of Iraq, Turkey, which has 20 million Kurds in its population of 80 million, has refused to be even slightly helpful to the Iraqi and Syrian Kurds; the Turks have been sitting on their hands in idling tanks just a few hundred meters from the Syrian Kurdish town of Khobani, which has been under intense attack by the Islamic State for several weeks.

Turkey, in its opposition to the beleaguered and discredited Assad regime in Syria, has effectively supported the IS, tolerating the dispatch of some supplies to it and denying the United States and its allies the right to use Turkish air bases as launching points for air strikes on it. As a result, the strikes must be launched from aircraft carriers, a manageable but serious inconvenience. (Egypt’s President el-Sisi declared last weekend that terrorism sponsored by the IS, or undertaken by IS imitators, “threatens the existence” of Egypt, after 31 people were killed in terrorist bombings in northeast Sinai, near the approaches to Gaza. Turkey should not be giving any comfort to such mutants as the IS.)

In this policy, Turkey has perversely taken the side of the most radically bellicose Islamic movement in history and leapfrogged over the positions of all the previous sponsors of less virulent terrorism: Iran, Saudi Arabia, Qatar, and Syria. Those countries, on this issue, are on the same side as Israel, and one of the few benefits of this latest ratchet-up of terrorist barbarism engineered by the IS is its revelation that militant Islamists see no practical differences between Christians, Jews, moderate Arabs, and even terroristic Islamists slightly less fanatical and unencumbered by the vestiges of civility than they. The tired old leftist claim that all problems would end if the West just put Israel over the side was always bunk, and now Palestinians and the Palestine question have effectively vanished from the radar screens of the Middle East, remembered only by a rag-tag of Israelophobic useful (to Hamas and the PLO) idiots in the West.

What Turkey should do is join with other civilized countries against all the terrorists, effectively establish a protectorate over Syria, and cooperate with Saudi Arabia, Egypt, and even Iran, if that country can be induced to abandon its nuclear military program, in stabilizing Iraq. The world should recognize an independent Kurdistan, one that would pledge not to disturb Turkey if Turkey stops its support or even toleration of Islamist terrorism; the Muslim powers should recognize the right of Israel to exist as a Jewish state (as Egypt, Jordan, and even, in theory, Turkey have); and a viable state can be crafted for Palestine involving a narrowed West Bank but a thickened Gaza, with a connection between them.

Ironically, the eruption of stateless, pan-national terrorist groups such as al-Qaeda and the Islamic State tends to push the Iranians, Arab governments, and Israel into a common front with the West and Russia, a potentially useful condition. If threatening to expel or suspend Turkey from NATO and to encourage the unhappy Turkish Kurds (who claimed on the weekend to have transformed the southeastern Turkish town of Cizre into an “autonomous zone”) will help bring Mr. Erdogan to his senses, those steps should be taken.

If they don’t bring him to his senses, the threats should be carried out. Allies must be given reasonable leeway, and Turkey has legitimate grievances against Europe, but treachery on Mr. Erdogan’s scale is intolerable, and if tolerated can only lead to greater betrayals and provocations. To be a member of the most successful alliance in history is a privilege, not a right, and this privilege must not be abused.

cbletters@gmail.com. From the National Review.