Mr. Hussein set the stage for war with Iran by repudiating a 1975 agreement that had settled a disputed over the Shatt al Arab, the strategic waterway along their border. According to Amatzia Baram, an Israeli expert on Iraq who has studied the archive, the pivotal decision appears have been made in a meeting on Sept. 16, 1980, when Mr. Hussein took the optimistic view that the Iranians, fearing the Iraqi forces massed near the border, would give in without much of a fight.

A top secret report from the Iraqi General Military Intelligence Directorate supported Mr. Hussein’s assessment. “It is clear that, at present, Iran has no power to launch wide offensive operations against Iraq or to defend on a large scale,” the report noted. It also predicted “more deterioration of the general situation of Iran’s fighting capability.”

But the war, which ultimately lasted eight years and resulted in hundreds of thousands of casualties, turned out to be far more difficult than Mr. Hussein had expected. Soon after it began, Iranian aircraft bombed a series of targets, including Iraqi oil refineries and the Osirak nuclear plant south of Baghdad. The feat so surprised the Iraqis that they assumed the attack could not have emanated from Iran.

“This is Israel,” Mr. Hussein exclaimed in an Oct. 1, 1980, meeting. He then complained that Iraqi officials had not followed his suggestion to bury the nuclear facility under the Hamrin Mountains north of Baghdad, before approving a plan to fortify the complex with millions of sandbags. But those sandbags proved to be of little use when Israeli warplanes actually did strike the site, in June 1981.

Later, Mr. Hussein said he was not surprised that Israel felt threatened by Iraq, which he asserted would defeat Iran and emerge with a military that was stronger than ever. “Once Iraq walks out victorious, there will not be any Israel,” he said in a 1982 conversation. “Technically, they are right in all of their attempts to harm Iraq.”

As Iraq’s war with Iran proceeded, Mr. Hussein did not hesitate to give battlefield advice, despite his shaky knowledge of weapons and tactics. “Do you have cannons that shell air bursts to fall on them while they are in the streets?” he said in the meeting on Oct. 1, 1980, which discussed the bombardment of Abadan, in southern Iran. “We want their casualties to be high.”

He was often cordial to his largely sycophantic inner circle, but was capable of coldhearted calculations about the forces he had sent to war. Early in the conflict, Mr. Hussein was frustrated with Iraqi bomber pilots who, hobbled by poor intelligence, had returned from missions over Iran after failing to strike their targets. Deciding that he needed to make an example of the airmen, Mr. Hussein demanded that the pilots be executed, a practice that former Iraqi commanders say was common during the war.