In 1975, Iran and Iraq negotiated the Algiers Agreement. It settled border disputes between the two countries. In March 1980, Saddam Hussein unilaterally abrogated it. Carter officials encouraged him.
Journalist/historian Dilip Hiro noted:
"According to the Iranian president, Bani-Sadr, in early August 1980 his government had purchased secret documents containing a detailed account of the conversations in France between several deposed Iranian generals and politicians, Iraqi representatives, and American and Israeli military experts."
"If so, the administration of President James Carter had an inkling of Iraqi plans. By supplying secret information, which exaggerated Iran's military weakness, to Saudi Arabia for onward transmission to Baghdad, Washington encouraged Iraq to attack Iran."
Saddam was supported by CIA-sponsored Iranian military officers given refuge in Iraq. Soviet Russia feared revolutionary Islam spreading to central Asia.
Saddam saw his chance to wage war and win. He hoped to defeat a regional rival, annex parts of Iran, and strengthen his regional position.
Washington wanted its own regional influence enhanced. The Carter Doctrine pledged Middle East military intervention if US interests were threatened.
According to columnist Jack Anderson, he considered invading Iran, seizing its oil fields, and boosting his electoral prospects, he hoped. Soviet Russia threatened intervention if he followed through.
Carter abandoned his plans. At the same time, his administration remained hostile to Ayatollah Khomeini's government.