He authored the so-called "Manning Memo", that summarized the details of a meeting between American president George W. Bush and British prime minister Tony Blair during the run-up to the invasion of Iraq in 2003.

During this time he developed a close relationship with his counterpart, then US National Security Advisor Condoleezza Rice.

During a private two-hour meeting in the Oval Office on 31 January 2003, Bush made it clear to Prime Minister Tony Blair of Britain that he, Bush, was determined to invade Iraq without the second resolution, or even if international arms inspectors failed to find unconventional weapons, stated a confidential memo about the meeting written by Manning and reviewed by The New York Times.

At their meeting, Bush and Blair candidly expressed their doubts that chemical, biological or nuclear weapons would be found in Iraq in the coming weeks, the memo said.

[1]His close relationship with the Prime Minister suggests he has been a key figure in driving British foreign policy in respect of the United States, particularly in the aftermath of the 11 September 2001 attacks and the decision to invade Iraq.

Six months later, he joined BG Group (formerly British Gas plc) on a part-time basis at a reported annual salary of £80,000.