It is necessary, and indeed a duty to recall the centenary of that tragic event

Pope Francis on Sunday referred to the 1915 mass killings of Armenians by Turks as the “first genocide of the 20th century,” entering into a tense historical debate with wider implications for the Vatican’s relations with Turkey and Islam. The Wall Street Journal reports.

With the statement, during a Mass in St. Peter’s Basilica in the Vatican to mark 100 years since the killings, the pope linked the event to contemporary persecution of Christians in the Muslim world, effectively turning the occasion into his latest denunciation of attacks on Christians by Muslim groups, including the April 2 killings at a university in Kenya.

Armenians say as many as 1.5 million Armenians were systematically killed during World War I in today’s eastern Turkey, which was then part of the Ottoman Empire.

Many countries officially recognize the killings as genocide. But Turkey contests Armenian claims about the scale of losses; it argues that hundreds of thousands actually died in warfare and famine, and that many Turks were also killed by Armenians. Turkey argues that the question of genocide should be left to historians rather than politicians.

Turkey’s Foreign Ministry summoned the Vatican’s envoy to Ankara following the pope’s comments, according to a Turkish official. The ministry will issue a statement later on Sunday, the official said, without providing additional details about the meeting with the Vatican representative.

Pope Francis said on Sunday that “it is necessary, and indeed a duty” to “recall the centenary of that tragic event, that immense and senseless slaughter whose cruelty your forbears had to endure...Concealing or denying evil is like allowing a wound to keep bleeding without bandaging it.”

Sunday wasn’t the first time a pope has referred to the 1915 deaths as genocide; the Vatican’s published version of the pope’s words cited a 2001 common declaration by Pope John Paul II and Catholicos Karekin II, head of the Armenian Apostolic Church, who was also present at Sunday’s Mass, along with Armenian President Serzh Sargsyan.

But Pope Francis went further than the 2001 declaration, calling the killing of Armenians one of “three massive and unprecedented tragedies” in the 20th century.

“The remaining two were perpetrated by Nazism and Stalinism,” he said. The latter reference was apparently to the 1932-33 man-made famine in Ukraine, part of Joseph Stalin’s effort to collectivize Soviet agriculture, which killed as many as 7.5 million.

The pope also set the 1915 event in a contemporary context, with an impassioned reference to “so many of our defenseless brothers and sisters who, on account of their faith in Christ or their ethnic origin, are publicly and ruthlessly put to death—decapitated, crucified, burned alive—or forced to leave their homeland.”

The persecution of Christians around the world, especially in Muslim-majority countries, has become an increasingly prominent and urgent theme in Pope Francis’ public statements. He has called on Muslim leaders to denounce the actions of Islamic State terrorists in Syria and Iraq.

At a Good Friday ceremony on April 3, he deplored the world’s “complicit silence” about such persecution, including the previous day’s killings of almost 150—many Christians—by a Somali militant group in Kenya.

The Good Friday ceremony prominently featured Christians from Iraq, Syria, Nigeria, Egypt and China, where Christians experience varying degrees of violence and official discrimination.

Sunday’s Mass commemorated the centenary of what Armenians call the “Metz Yeghern,” a term that Vatican translators rendered as “Great Evil.” It was also the occasion for Pope Francis to proclaim St. Gregory of Narek a “doctor of the church,” honoring the 10th-century Armenian monk for his contributions to theology.

The pope also called for reconciliation between Turkey and Armenia, but his words are likely to raise tensions between the Vatican and Ankara.

Asked about reports that Ankara had summoned the Vatican’s envoy, Vatican spokesman Rev. Federico Lombardi declined to comment.

Pope Francis had referred to the killing as a genocide before his election as pope, but had lately treated the issue more diplomatically.

Speaking to reporters at the end of a November trip to Turkey, the pope praised a statement made last April by Turkish leader Recep Tayyip Erdogan—then prime minister and president since August—that offered “condolences” to descendants of the Armenian dead and expressed hope for “compassion and mutually humane attitudes towards one another.”

Mr. Erdogan has repeatedly called for a joint historical commission to establish a definitive record of the events, seeking to settle a century of mutual recriminations between Turks and Armenians.

But the Turkish president’s rhetoric has recently shifted, with Mr. Erdogan accusing the Armenian diaspora and Western leaders of fomenting enmity between the two peoples with genocide accusations.

As the 1915 anniversary nears, the geopolitics of the massacres are being played out in Washington and capitals of nations where the Armenian diaspora have made their homes. A group of 40 Congressmen in March introduced a resolution to formally recognize the Armenian genocide, a move likely to strain U.S.-Turkish relations.

President Barack Obama pledged during his 2008 campaign that he would formally recognize the genocide, but he has subsequently backed away from that commitment.