Does it matter that Turkey appears to think that it can relitigate the outcome of World War I? That’s the question observers were forced to confront last week when its president, Recep Tayyip Erdoğan, opened that country’s parliament with a speech about the status of Jerusalem.

The Islamist government Mr. Erdoğan heads is among the leading boosters of the Palestinian war against Israel’s existence, as well as an ally of the Hamas terrorists in Gaza. The Turkish leader’s remarks, though, weren’t framed as a response to American recognition of Jerusalem as Israel’s capital or the peace and normalization agreements reached between the Jewish state and the United Arab Emirates and Bahrain. Rather, he offered an argument that Jerusalem belongs to the Turks, rather than the Jews or the Arabs.

Neither Israel nor the United States is worried that Turkey will try to implement this absurd ambition. Yet the Erdoğan government’s recent moves, coupled with its outrageous statements, do call into question the Trump administration’s apparent belief that Turkey can or should be encouraged to continue on its present course.

The Jerusalem speech is — like Turkey’s alleged role in encouraging a renewal of fighting between Azerbaijan and Armenia, and its aggressive attitude towards the efforts of Greece, Cyprus, and Israel to work together on natural gas exploration in the Mediterranean — a signal that can’t be ignored. Either Mr. Trump or former Vice President Joe Biden will need to be as focused on the threat from Turkey in the future as they are on Iran.

Mr. Erdoğan’s Jerusalem claim was a reference to the fact that the Turkish Ottoman Empire governed the land of Israel for centuries. The Turks were evicted from Jerusalem and the rest of the country by Great Britain during the First World War. The Ottomans had picked the wrong side by allying themselves with Germany, and at the conflict’s conclusion, the British and French victors were able to carve up their vast empire.

Among other things, it left the British in possession of a League of Nations Mandate for Palestine in which they were charged to make good their Balfour Declaration promise to assist with the building of a national home for the Jewish people.

Stripped of their imperial glory and pretensions to be the caliphate of all Muslims, the Turks were confined to the Anatolian Peninsula, where they created a secular republic. Under the initial leadership of Mustafa Kemal Atatürk, the Turks focused on modernizing their country. After being smart enough to stay out of World War II, Turkey aligned itself with the West in opposition to its traditional Russian foes and joined the NATO alliance. As part of that Western and secular orientation, Turkey also became, in 1949, the first Muslim nation to recognize the State of Israel.

In the last two decades, under the leadership of Mr. Erdoğan and his Islamist AKP Party, Turkey has discarded both secularism and the West. Though it has never completely ruptured relations with Israel, the Turkish Islamist has become a cheerleader for Hamas and a constant source of irritation to Israel.

As a matter of practical policy, Mr. Erdoğan’s speech is ludicrous. The notion that Turkey can have anything to say about Jerusalem’s future is a joke. Yet neither should it be entirely ignored. Mr. Erdoğan’s focus on Jerusalem and the Temple Mount is a reminder of his decision to turn Istanbul’s Hagia Sophia back into a mosque.

The Hagia Sophia is a Byzantine Christian Cathedral that was converted to a mosque after the conquest of Constantinople, which the modern Turks renamed Istanbul. The point of that decision was to reinforce Mr. Erdoğan’s claim to be the leader of the Muslim world — in effect, the head of a revived caliphate that would govern the whole region and its holy places in Saudi Arabia and Israel.

While no one is clamoring for a revival of the late unlamented Ottoman Empire, Mr. Erdoğan’s pose as the leader of all Muslims does prop him up at home and serves as the backdrop for Turkey’s increasingly aggressive foreign policy.

The Turks have been using their military to threaten the efforts of Greece, Cyprus, and Israel to work together on natural gas. They’ve also been throwing their weight around elsewhere, establishing a zone of control in northern Syria with American acquiescence and pushing the Azeris to settle scores with the Armenians, who seized territory from them in the 1990s.

Mr. Trump was severely criticized for going along with Turkey in Syria in order to enable him to start withdrawing American troops from the region. But, contrary to my expectations, this seeming betrayal of the Kurds who had fought alongside the United States against ISIS terrorists didn’t lead to total disaster. Still, the American move has, along with Mr. Trump’s seeming embrace of Mr. Erdoğan, encouraged the Turks in their other provocative behavior.

The American decision to go soft on Turkey was motivated by a desire to keep Mr. Erdoğan from forming an alliance with Russia and because the United States has been rightly focused on isolating Iran, whose own quest for regional hegemony and nuclear ambitions are the No. 1 threats to peace and stability.

The entangling alliances in the region are confusing and contradictory. Israel is friendly with the Azeris, who also look to Turkey as an ally, while the Armenians have gotten some support from Iran. Turkish intervention in Libya, which wasn’t discouraged by Mr. Trump, is of concern to Israel. Meanwhile, President Putin’s desire to revive the old Soviet empire looms over all these problems as a constant threat.

Even more importantly, Turkey has sought to sabotage peace deals between Gulf state Arabs and Israel, something that is one of the Trump administration’s greatest achievements.

While the Trump administration has done well in focusing on stopping Iran, its Turkey policy has been a confusing mess exacerbated by the president’s foolish boasts about commanding Mr. Erdoğan’s respect. To the extent that Mr. Biden has a position on these issues, it involves being tougher on Turkey, but also includes a return to President Obama’s feckless appeasement of Iran.

What the United States needs is to realize that it’s possible to continue to quarantine the dangerous Islamists of Tehran without allowing an authoritarian megalomaniac like Mr. Erdoğan to think he has America’s blessing to be just as disruptive a force as the Iranians. Both Iran and Turkey have each used the rallying cry of taking Jerusalem from the Jews to justify their insane and dangerous goals.

A rational American foreign policy will not involve being suckered by either Iran or Turkey. It remains to be seen if either Messrs. Trump or Biden is capable of that kind of clarity.

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From the Jewish News Syndicate, of which Mr. Tobin is editor-in-chief.