Iran is never mentioned when President George W. Bush explains what led to his surprising acceptance of a "time horizon" for the withdrawal of American combat forces from Iraq. But I found clear evidence on a recent visit to Tehran that Iranian diplomacy was a key factor behind his reversal.

The White House explanation is that the success of the administration's "surge" policy is enabling Iraq to stand on its own, releasing pent-up nationalist opposition to the presence of foreign soldiers.

This, in turn, it is said, has compelled the Iraqi prime minister, Nuri Kamal al-Maliki, to demand inclusion of a withdrawal timetable in a projected Iraq-U.S. security agreement so that his opponents cannot use nationalism against him in the fall provincial elections.

What this explanation omits is the crucial role that Iran has played in Maliki's conversion. It was only after Iranian intervention, I learned in Tehran, that Maliki shifted to his newly tough stand in the deadlocked negotiations with Washington on the security agreement.

When a draft U.S.-Iraq accord without a time table was signed on March 17, it remained a well-kept secret until nationalist critics within Maliki's inner circle leaked it in early May to Iranian diplomats and to the Iraqi media. The reaction in Tehran was explosive. On May 11, Hossein Shariatmadari, editor of the hard-line daily Kayhan, attacked it in a vitriolic signed editorial entitled "Iraq on the Edge" that he handed to me during an hour-long interview.

"If you want to know what has been happening," he said, "I suggest you read this."

Shariatmadari is the "personal representative of the Supreme Leader," Ayatollah Ali Khamenei, and is widely regarded as his media spokesman.

"How is it," the editorial asked, "that the Maliki government took the first steps toward signing such a disgraceful pact in the first place?"

The United States, it said, is using the treaty to "sow the seeds of discord" between Maliki and his coalition partner, Abdel Aziz al-Hakim, so that "the U.S. can put pro-American individuals in charge. It is amazing that al-Maliki failed to see such a conspiracy coming."

In a clear warning to Maliki, the editorial added that if the treaty is implemented, Iraqis will replace his government with "another Islamist government."

Maliki was summoned to Tehran for a three-day dressing down from June 7 to 9 that led to his announcement on June 13 in Jordan that the negotiations with the United States had reached "a dead-end and deadlock."

Informants in government-affiliated think tanks told me that he had "difficult" meetings, as one put it, with Khamenei and with the Revolutionary Guard generals who oversee Iraq policy. Soon thereafter, his defense minister signed a secret mutual security accord with his Iranian counterpart.

Iran's first deputy foreign minister, Alireza Sheikhattar, repeatedly emphasized the importance of a firm withdrawal timetable. "We don't expect that U.S. forces can leave in a fortnight," he said, "but whether it's three months or eight months or longer, the important thing is a serious intent to withdraw gradually."

Asked if any American forces could remain, he replied, "yes, some could stay to help with training Iraq forces, if their goal is truly training," but Iraq "would not allow" the continued operation of U.S. air bases that could make Iraq "a platform for harming the security of Iran and other neighbors. Why should the United States operate air bases in Iraq?"

Maliki is increasingly upset, Sheikhattar added, that the United States "still exercises complete control over Iraqi airspace. The Iraqis should have a real air force of their own. Why are they prohibited from having more than token aircraft and related facilities, even for civil aviation? They are not poor. They can purchase fighters and have their own aircraft for both internal and external security."

Wouldn't this pose a potential security threat to Iran? Not if Iraq has a sovereign, democratic government, he said, "because there is an absolute majority in favor of Iran" now that the Shiite government is in control.

As if in reply to Sheikhattar, Admiral William Fallon, the recently retired commander of the U.S. Central Command, emphasized in an article published in The New York Times last week that "control of Iraqi air space" would be an "important component of the security agreement that would require clear headed negotiations."

Whether or not Baghdad actually does ask the United States to shut down its air bases, as Sheikhattar demanded, the next administration in Washington should consider removing U.S. long range bombers from Iraq.

Cooperation between Washington and Tehran is necessary for an orderly departure of American forces and for the economic stabilization of Iraq.

More important, no government in Baghdad is likely to survive for long if it ignores the legitimate security concerns of a powerful neighbor with a 1,000 mile common border and intimate historical, economic and Shiite religious ties that go back for a millennium.

Selig S. Harrison, director of the Iran program at the Center for International Policy, covered Iran for the Washington Post. He revisited Tehran in June of 2007 and February and June of 2008.