After the attacks on the US on 11 September 2001 and the fall of the Taliban regime in Afghanistan in November, the US Administration turned its attention to regime change in Iraq as part of the second phase of what it called the Global War on Terror.
The UK Government sought to influence the decisions of the US Administration and avoid unilateral US military action on Iraq by offering partnership to the US and seeking to build international support for the position that Iraq was a threat with which it was necessary to deal.
In Mr Blair's view, the decision to stand "shoulder to shoulder" with the US was an essential demonstration of solidarity with the UK's principal ally as well as being in the UK's long-term national interests.
To do so required the UK to reconcile its objective of disarming Iraq, if possible by peaceful means, with the US goal of regime change. That was achieved by the development of an ultimatum strategy threatening the use of force if Saddam Hussein did not comply with the demands of the international community, and by seeking to persuade the US to adopt that strategy and pursue it through the UN.
President Bush's decision, in September 2002, to challenge the UN to deal with Iraq, and the subsequent successful negotiation of resolution 1441 giving Iraq a final opportunity to comply with its disarmament obligations or face serious consequences if it did not, was perceived to be a major success for Mr Blair's strategy and his influence on President Bush.
But US willingness to act through the UN was limited. Following the Iraqi declaration of 7 December 2002, the UK perceived that President Bush had decided that the US would take military action in early 2003 if Saddam Hussein had not been disarmed and was still in power.
The timing of military action was entirely driven by the US Administration.
At the end of January 2003, Mr Blair accepted the US timetable for military action by mid-March. President Bush agreed to support a second resolution to help Mr Blair.
The UK Government's efforts to secure a second resolution faced opposition from those countries, notably France, Germany and Russia, which believed that the inspections process could continue. The inspectors reported that Iraqi co-operation, while far from perfect, was improving.
By early March, the US Administration was not prepared to allow inspections to continue or give Mr Blair more time to try to achieve support for action. The attempt to gain support for a second resolution was abandoned.
In the Inquiry's view, the diplomatic options had not at that stage been exhausted. Military action was therefore not a last resort.
In mid-March, Mr Blair's determination to stand alongside the US left the UK with a stark choice. It could act with the US but without the support of the majority of the Security Council in taking military action if Saddam Hussein did not accept the US ultimatum giving him 48 hours to leave. Or it could choose not to join US-led military action.
Led by Mr Blair, the UK Government chose to support military action.
Mr Blair asked Parliament to endorse a decision to invade and occupy a sovereign nation, without the support of a Security Council resolution explicitly authorising the use of force. Parliament endorsed that choice.