The infamy of the crimes against humanity carried out by Islamic State is legendary, but the still untold story is how Turkish President Recep Tayyip Erdogan deliberately turned a blind eye to the Islamic State’s use of his nation as a staging ground for attacks, and how he and his family benefited from illicit oil trading with the terrorists in Syria and Iraq. The president knows well that the evidence of his culpability is mounting, which likely explains his frenetic effort to win a referendum on April 16 that will grant him immunity from prosecution.

If Mr. Erdogan had not allowed the Islamic State operatives inside Turkey to transport weapons and explosives to Syria and Iraq openly, there would be no Islamic State as we know it today, and thousands of people likely would have evaded their brutality. By any legal standard, Mr. Erdogan has earned the label of “terrorist” himself. He has committed crimes against humanity.

The president’s missteps began in 2011, when he apparently surmised that the Syrian uprising was an opportunity for his regional and political interests. Initially, he aimed for a prompt regime change in Syria with this support. Eventually, as the Islamic State started to target the Kurds in the region, he considered the Islamic State as a tool that would help him conquer the Kurdistan Worker’s Party and deal with PKK’s Kurdish allies in Northern Syria. The Turkish State, under the direction of Mr. Erdogan, committed the following crimes against humanity:

Mr. Erdogan allowed more than 25,000 Islamic State foreign fighters to cross from Turkey to Syria and Iraq to join the terrorist organization, according to a recent report of the Combatting Terrorism Center at the U.S. Military Academy[1]. The Turkish intelligence and law enforcement forces were strictly ordered not to intervene with the movement of foreign fighters until 2016. In fact, in some cases, foreign fighters were accompanied by the Turkish intelligence officers.

Various Salafist Jihadist terrorist groups, including the al Qaeda affiliates and the Islamic State, were either provided arms and explosives by some elements of the Turkish State or were openly allowed to carry out their armament and logistical support operations inside and through Turkey[2]. For example, on Jan. 19, 2014, three trucks operated by the Turkish National Intelligence (MIT) loaded with hidden military grade weapons underneath medicine boxes were stopped on Adana highway, revealing the ongoing transfer of arms and explosives to terrorists in Syria[3].

Turkey had been the main supplier of arms and explosive materials for the Islamic State, according to the December 2016 report of Conflict Armament Research (CAR), a research organization funded by the European Union to identify and track the conventional weapons supply chain to the Islamic State. “CAR’s findings continuously reinforce evidence that the Islamic State operates a major acquisition network in Turkey and has a direct line of supply from Turkey, through Syria, to the Mosul area,” CAR reported[4].

As the former chief of counterterrorism police in Turkey in 2013, I personally witnessed that hospitals across southeastern Turkey opened their doors to any wounded Islamic State fighter and treated him free of charge, often under the protection of Turkish police[5]. While I was the chief, I was ordered to provide security to the wounded fighters — and there were so many of them; even if I wanted to provide security, I would not able to find enough officers to assign. I was personally assigning the security details by orders of the governor. In fact, following an airstrike, Abu Bakr al­-Baghdadi’s right hand man and six other fighters were treated in August 2014 in a private hospital in Sanliurfa. The Turkish government paid the hospital bills, according to hospital doctors who spoke to me.

The world learned through the hacked emails of Berat Albayrak, Turkey’s Minister of Energy and Natural Resources and the son in law of Mr. Erdogan, that he continuously purchased Islamic State oil through Powertrans, his front company for illicit oil transfers. His purchases provided the terrorist organization a steady income of $3 million a day at the height of Islamic State’s oil production — thereby offering the terror network hundreds of millions of dollars to fund its operations[6].

Mr. Erdogan released into the custody of Islamic State on two separate occasions more than 200 hard­core terrorists in order to gain concessions from ISIS. In all, 201 Islamic State fighters who were residing in Turkey were arrested by Turkish police for the chief purpose of bartering them for Turkish officials held hostage by ISIS in Iraq, or to gain the release of Turkish soldiers stranded and encircled by ISIS fighters in Syria.

In order to get the release of approximately 49 Turkish consular officials kidnapped by ISIS after the seizure of the Turkish Consulate in June 11, 2014, the Turkish police rounded up more than 100 ISIS members sheltering on Turkish soil and offered them to ISIS as a prisoner exchange on Sept. 20, 2014.

The second exchange happened during the Suleyman Shah tomb operation on Feb. 22, 2015, in the town of Sarrin in Aleppo Province. In that operation, Turkey handed 98 ISIS members to the terrorist organization, so that the tomb could be moved. The disturbing part of these exchanges is the fact that the State of Turkey handed over several well-known and experienced foreign fighters to the Islamic State, including some who had “red notices,” that is, arrest warrants from Interpol.

For example, Ahmed Diini, a Dutch citizen of Somali origin who was wanted by American authorities on terrorism charges, was handed to the Islamic State on Sept. 20, 2014, even though the U.S. government had requested that Turkey extradite him back to the U.S. for trial.

Similarly, Islamic State senior leader Mohamed Mahmoud, known as Abu Usama al­-Gharib, an Austrian citizen, was among the terrorists who were handed over to the Islamic State. Mahmoud went on to command large units of ISIS terrorists in Syria and reportedly murdered hundreds of people.

The Islamic State freely operated in Turkey for years without facing any counterterrorism operations. After sensational corruption scandals surfaced in 2013 that appeared to implicate Erdogan’s son, Bilal, the president called a halt to all ongoing counterterrorism operations against the Islamic State, the al Qaeda and its affiliates. In 2104 and 2015, Turkish police neglected to carry out any planned counterterrorism operations in Turkey targeting those terrorist organizations[7]. It appeared that the police realized that Islamic State and other jihadist groups were untouchable. The whole nation was put on notice on Jan. 15, 2014, when police chiefs and prosecutors from Van, a city in the east of Turkey, attempted to interdict arms smuggling from Turkey to Syria — but were instead promptly fired and arrested themselves.

Whereas 10 years ago, all Turkish parties took a hard line against radical, political Islamism, Mr. Erdogan’s Justice and Development Party (AKP) in recent years began to praise the mujahidin as freedom fighters, which justified their presence on Turkish soil. A 2015 Pew Research Center survey confirmed this tendency by reporting that approximately 7 million of Turks had a favorable approach towards the Islamic State[8].

Journalists and scholars point to the president’s Islamist rhetoric and the AKP’s lapdog media platforms for radicalizing many Turks, a tendency illustrated by the horrendous assassination of the Russian ambassador to Ankara on camera by a police officer on Dec. 19, 2016[9].

Islamic State in today’s Turkey enjoys mind­boggling privileges: It has opened four schools in the middle of Ankara, maintained training centers and camps in Istanbul, and operated hundreds of safe havens and terrorist cells across the country.

Mr. Erdogan and his government systematically, willingly, and in most cases openly supported Salafist terrorist organizations, as documented by Turkish independent media (before the government closed them) and European news organizations.

The grim result of this support is that Islamic State became a monstrous killing machine in a very short time, killing thousands of men, women and including children. Even worse, atrocities unseen since World War II were committed: Men and women were beheaded or killed, or burned, tortured, raped and forced to move because of the Islamic State in Syria and Iraq. Mr. Erdogan has the blood of innocent people on his hands — in fact, more so than many ISIS foot soldiers, because atrocities were enabled on his watch while he paid no attention.