On 20 March 2003, as bombs and missiles from the US-led coalition rained on Iraqi cities in the opening “shock and awe” campaign against Saddam Hussein, the tectonic plates of the post-Second World War international order shifted permanently. For those of us who covered the run-up to the war, the invasion and the long aftermath of an occupation marked by the country’s violent sectarian fracturing, the profound consequences of that momentous day were obscured by the shock of the unfolding events. When US President George W Bush delivered his “Mission accomplished” speech on 1 May, a whiff of hubris was already evident in a country racked by looting and where destabilising struggles for power were emerging. What we could not understand then was the scale of the reckoning to come. Looking back, I recall the jubilation among those who backed the invasion at how easily it had seemed to go. The naysayers had been proved wrong. Saddam and his brutal regime were gone in what was lauded as a brief and model military operation. US arms appeared pre-eminent. It was a chimera. Two weeks ago, I returned from the frontline of another full-scale and brutal conflict: Russia’s war against Ukraine. Without diminishing Russian President Vladimir Putin’s agency in the crimes he is committing there, Russia’s aggression against Ukraine would not have been possible without Iraq. I am not arguing that Ukraine is a direct consequence of Iraq. The moral equivalences – where they are detectable – are far more complex than Putin apologists claim in whataboutery appeals pointing to Iraq. As Patricia Lewis, head of the international security programme at Chatham House told me last week, ahead of an event on Monday tracing the two decades from Iraq to Ukraine: “It wasn’t a straight line. There are other paths that could have been taken. But it was a massive own goal. When Putin talks about weapons of mass destruction in Ukraine, part of it is to remind people of Iraq.” Because the invasion of Iraq unquestionably created a space for a bad actor such as Putin to challenge one of the most essential elements of modern international law regarding conflict: that states should not acquire territory by conquest. However you judge the motives of Bush and the then British prime minister, Tony Blair, who led the charge to invade – foolish, venal, messianic or self-serving – their tearing up of the rules-based international order to launch an intervention based on misinformation established a precedent that would be exploited by Moscow and others. The long, bloody years of occupation – the suicide bombings, insurgent groups and the death squads – had their own chain of consequences. The vulnerabilities of US-led western military power, exposed in Iraq but also in Afghanistan, punctured Washington’s post-cold war claim to being the sole superpower in a unipolar world. The longer “Iraq effect” was more pernicious. It described a deepening debilitation on the international stage that was the result of both tarnished moral authority and creeping war fatigue. When the west was tempted again into another intervention in Libya, there were no ground forces. Later, confronted by his own red line in Syria over chemical weapons use in 2013, and with Russian warships at sea, President Barack Obama blinked. There was no meaningful response. As Oxford professor Louise Fawcett noted recently, the invasion can be seen as a “critical juncture” – a historic moment that reshapes the pre-existing order – in the same way as Eric Hobsbawm’s “short 20th century” was bracketed by the outbreak of the Great War and the dissolution of the Soviet Union. And while the invasion was sold by neoconservatives around Bush as a way to remake the Middle East as more stable and democratic, the result was often the opposite. There is a credible argument that, in Tehran, Saddam’s fall was seen as an impetus, rather than a discouragement, for nuclear enrichment, as it also lit the spark for the Islamic State insurgency and exacerbated long-festering tensions between Iran and Saudi Arabia. Not everything can be laid at the feet of Bush and Blair: not least, the coincidence of the post-invasion period with the rise of China and rebirth of Russian imperial ambitions under Putin, as well as Iran’s efforts to expand its regional influence. But, in different ways, those actors took the invasion as a reference point to push back against the west, invoking the notion of western hypocrisy as cover for their own ambitions, and to challenge what was legitimate under international law. Perhaps the biggest fallout was the one that was least visible at first. The resurgence of rightwing US isolationism under Donald Trump (who, in a 2016 candidates’ debate, described the war in Iraq as a “big, fat mistake”) would be tracked and analysed and gamed by the west’s opponents. While it is impossible to know the detail and scope of those calculations, the aggressive brinkmanship that has been visible in a series of security crises in the past decade or so is highly suggestive. From the use of chemical weapons in Syria (where Russia and Iran intervened on the side of the murderous Assad regime) to Yemen, Taiwan and the South China Sea, to Russia’s annexation of Crimea in 2014 and North Korea’s nuclear and ballistic weapons programme, a new sense of impunity has coalesced into ever more obvious military alignments between several of these states. Finally, it is perhaps worth returning to the lies that led to the Iraq war. The manipulation of intelligence and disinformation occurred in a pre-social-media era and before the normalisation of political lying under Trump and Boris Johnson. But the Iraq war fabrications can be seen as a start point in a new period of widespread state-sanctioned misinformation, in which China and Russia have become the two most prominent actors. Two decades later, we are still counting the cost. Peter Beaumont covered the war in Iraq for the Observer and Guardian, winning the Orwell prize and an Amnesty award for his reporting