What happened
The administration's public case for war rested on two claims: that Iraq had an active weapons of mass destruction programme, and that Saddam Hussein's government was linked to the al-Qaeda network responsible for the September 11 attacks. Both were central to Secretary of State Colin Powell's February 2003 presentation to the UN Security Council, and both collapsed under subsequent scrutiny — the CIA-led Iraq Survey Group's 2004 Duelfer Report found no viable WMD programme, and the bipartisan 9/11 Commission found no credible evidence of an operational Saddam–al-Qaeda relationship.
The war's costs, by any measure, dwarfed its stated justification. Estimates of the total US expenditure run to roughly $2 trillion over the war's duration. Approximately 4,500 US service members were killed and 32,000 wounded. Iraqi civilian deaths are far harder to pin down precisely — the Iraq Body Count project's conservative documented-incident methodology puts the figure above 300,000, while population-survey-based studies including the Lancet's have produced estimates as high as 650,000 to over a million including indirect, excess deaths from the collapse of health infrastructure and years of subsequent sectarian violence.
The invasion also destabilised the wider region in ways that persisted for two decades, dismantling Iraqi state institutions in a manner that created the conditions for the sectarian civil war of 2006–2008 and, later, the rise of ISIS.