A perceived turning point was when Blair controversially allied himself with US President George W. Bush in supporting the Iraq War, which caused him to lose much of his political support. The UN Secretary-General, among many, considered the war illegal. The Iraq War was deeply unpopular in most western countries, with Western governments divided in their support and under pressure from worldwide popular protests. The decisions that led up to the Iraq war and its subsequent conduct are currently the subject of Sir John Chilcot's Iraq Inquiry.

Who did Tony Blair side with?