Whom should we blame for the Islamic State? In the debate about its origins, many have concluded that it arose from the American-led coalition’s errors after the 2003 invasion of Iraq. In fact, the groundwork for the emergence of the militant jihadist group was laid many years earlier by the government of Saddam Hussein.

The Arab nationalist Baath Party, which seized power in 1968 in a coup in which Mr. Hussein played a key role, had a firmly secular outlook. This held through the 1970s, even as religiosity rose among the Iraqi people. But soon after Mr. Hussein invaded Iran in 1980, it began to change.

In a few tactical instances during the 1980s, Mr. Hussein allied with Islamists, notably the Muslim Brotherhood, to destabilize his regional rival in Syria, but these were limited, plausibly deniable links. In 1986, however, the Pan-Arab Command, the Baath Party’s top ideological institution, formally reoriented Iraq’s foreign policy toward an alliance with Islamists. This was the first clear deviation from secular Baathism.

The shift was accompanied by a domestic “Islamization,” with regime media dropping references to a “secular state” and describing the war against Iran as a “jihad.” The changes accelerated after 1989 when Michel Aflaq, the Christian founder of the Baath Party, died, and Mr. Hussein claimed that Mr. Aflaq had converted to Islam. Alive, Mr. Aflaq was a bulwark against Islamization; as a dead convert, he could — and did — baptize a new direction.

The campaign of Islamization intensified further after Iraq’s devastating defeat in Kuwait in 1991 and the subsequent Shiite revolt, culminating in 1993 with Mr. Hussein’s abandonment of the last vestiges of Baath secularism when he initiated the Faith Campaign. In some respects, Mr. Hussein’s government was following rather than leading public opinion, as Iraqis fell back on their faith for solace under the harsh international sanctions. But what began as a cynical attempt to shore up support, as the regime retreated to its Sunni tribal base, took on a life of its own, transforming Iraq into an Islamist state and imposing lasting changes on Iraqi society. ...