This isn’t how members of the same club are supposed to behave, is it? At a press conference following the conclusion of the NATO summit at Madrid today, Turkey’s president blasted fellow alliance member Greece, leveling in unambiguous language the spurious and inflammatory charge that Athens is undertaking military provocations against Ankara. 

The specific target of President Erdogan’s rage was the level-headed Greek prime minister, Kyriakos Mitsotakis, who he accused of “going to Congress and making a speech where he simply reversed” an agreement to settle some issues bilaterally. 

That was a reference to Mr. Mitsotakis’s appearance before a joint session of Congress in May, and the issues hinge on revisionist Turkish claims to some elements of Greek sovereignty over a number of islands in the Aegean Sea. “Greece has adopted a behavior that is not in line with the way we govern,” Mr. Erdogan said. “After lunch in Istanbul [in March], I told Mitsotakis to have bilateral relations and not to put third parties in between.” The “third party” in this case is the United States.

The specific issue is Ankara’s stubborn position that several Greek islands including Samos should not be militarized in any way, even though they constitute Greek territory. Mr. Erdogan has repeatedly evoked arcane language in the 1923 Treaty of Lausanne, which Athens considers a willful distortion and provocation from a fellow member of NATO. 

At Madrid, Mr. Erdogan also criticized Greek-American defense cooperation with respect to deterring further Russian aggression. In recent weeks he has openly characterized Greek military bases in Greece as “American,” and with characteristic bluntness he said today: “Our people do not swallow that this is for Russia.”

To make matters worse, the leader of the rogue mini-state of northern Cyprus, which is occupied by Turkish troops, today accused Greece of seeking to annex the island of Cyprus. “The real intention of Greece is to make Cyprus its 13th island,” Ersin Tatar told Turkish daily Hurriyet.

Greece has several hundred islands, so Mr. Tatar’s choice of the number 13 is a clear reference to the Dodecanese, a Greek archipelago in the Aegean Sea consisting of a dozen main islands including Kos and Rhodes. The Turkish Cypriot politician answers to the president of Turkey, and this is the kind of heated rhetoric that preceded events in the early 1970s that led to a Turkish invasion of Cyprus. 

Mr. Mitsotakis, also speaking at Madrid, had words of his own. Responding to some of his Turkish counterpart’s escalatory remarks, the Greek premier said, “We have to meet, we have to talk and we have to deal with the situation in a civilized way, in the context of international law.” He also said the Greek government would “react” to a new Turkish campaign to trademark the manufactured word “Turkaegean” for use in an international tourism marketing campaign. Greece’s minister for development and investments, Adonis Georgiadis, has ordered an investigation into the issue. 

Tensions with Turkey could portend a hot summer in the Aegean Sea, particularly if Ankara decides to match its revisionist rhetoric with military maneuvers in coastal waters that skirt Greek territory. Unauthorized overflights of Greek airspace by Turkish jets already occur on an almost daily basis. It is not clear whether these pose any risk to commercial aviation over the Aegean Sea, which is home to many popular holiday islands.

Yet the verbal gymnastics between Messrs. Mitsotakis and Erdogan do not sit particularly well with the Greek public, and decreasingly so for one opposition leader and former prime minister, Alexis Tsipras. Speaking in the Hellenic parliament while the NATO summit was under way near the other side of the Mediterranean, Mr. Tsipras called on Mr. Mitsotakis to demand that NATO “make a formal commitment to support Greece against Turkey’s expansionist aspirations and challenges,” according to the Greek press agency AMNA. 

Turkey “is rapidly upgrading its role and status in international developments following the Russian invasion in Ukraine, while also sharpening its aggressive rhetoric and its publicly expressed revisionist ambitions in the Aegean,” Mr. Tsipras said. In a reference to next year’s elections in Greece, he also said that Mr. Mitsotakis was making Greek foreign policy “fully subservient to the propaganda of an electoral campaign,” which, in Mr. Tsipras’s view, “encourages Ankara to persist with its behavior.” 

If the NATO command sees Turkish behavior as undercutting alliance cooperation on Ukraine in any way, expect admonishments to come from Washington and Brussels in the weeks ahead — formally or, more likely and as some reportedly already have, hushed.