The Iraq War began with the US-led 2003 invasion of Iraq. The Government of Canada did not at any time formally declare war against Iraq, and the level and nature of this participation, which changed over time, was controversial.
While Canada had previously participated in military action against Iraq in the Gulf War of 1991, it refused to declare war against Iraq without United Nations approval. Even so, Prime Minister Jean Chrétien said on 10 October 2002 that Canada would, in fact, be part of a military coalition to invade Iraq if it were sanctioned by the United Nations. However, when the United States and the United Kingdom subsequently withdrew their diplomatic efforts to gain that UN sanction, Jean Chrétien announced in Parliament on 17 March 2003 that Canada would not participate in the pending invasion. Nevertheless, he offered the US and its soldiers his moral support. However, according to classified U.S. documents released by , a high-ranking Canadian official may have secretly promised to clandestinely support the invasion. Two days earlier, a quarter million people in Montreal had marched against the pending war. Major anti-war demonstrations had taken place in several other Canadian cities.
Canada's relation to the Iraq War that began in 2003 was unlike Canada's role in the 2001 invasion of Afghanistan because it was far less direct. About a hundred Canadian exchange officers, on exchange to American units, participated in the invasion of Iraq. It has been reported that Canadian troops in the region numbered fewer than only three other participating countries. The War also affected Canada in the form of protests and counter-protests related to the conflict, and United States Military members who sought refuge in the country after deserting their posts to avoid deployment to Iraq—but who, unlike as with the Vietnam War, were mostly returned by Canada forthwith to the United States.