That was the verdict of Sir Michael Wood, senior legal adviser at the Foreign Office in 2003. And that was the advice he gave, in writing, to the then foreign secretary, Jack Straw.
So how did Mr Straw, and the then prime minister, Tony Blair, come to believe that such an invasion would be not just a crime of aggression, but that it would be perfectly legal?
This week, the Chilcot Inquiry into government decision-making in the run-up to the invasion has shed some light on this mysterious 180-degree shift in opinion on the legality of the war.
What happened after Sir Michael offered that advice was this. Mr Straw noted the advice, but disagreed with it. Advice was then sought from the most senior legal adviser to the government, attorney general Lord Goldsmith.
Newly declassified documents show he was initially sceptical that an invasion of Iraq without a second specific Security Council resolution authorising it would be legal. But he, along with a slew of other figures at the top of government, eventually changed their mind. Lord Goldsmith eventually ruled it was lawful, and the rest is history.
There has been much speculation on the reasons why so many figures at the top of government changed their mind. There is one that has, so far, has not been fully appreciated. It is simply this: they went to the United States. Lord Goldsmith changed his mind about 48 hours after visiting Washington. Why did this make such a big difference? There are three main reasons.
First, because Lord Goldsmith and others like him in government were dazzled by the pomp and power of the world's last remaining superpower. They would have seen the lavish accoutrements of US power and felt the reverence with which a popular president is held, as head of state as well as head of government.
In my view that this is also the reason – and a bad reason – why Mr Blair did not cut a better deal with George Bush. He too was dazzled and flattered by the attention he received in the US.
These UK politicians would have also sensed the determination in the ruling circles in Washington that an invasion was a done deal. The question in the White House was not "will it happen?" but "will we have to go it alone?"
But I think there was another factor which would have made the US trip change the minds of Mr Blair and his colleagues. The culture among most of the ruling Republicans at the time – including vice-president Dick Cheney, defence secretary Donald Rumsfeld and UN ambassador John Bolton – was particularly scornful of international law. They represented a significant constituency in the US that regards international law as a flimsy construct subject to the political manipulation of world leaders, the UN as an aid agency with ideas above its station, and the idea of the legality or illegality of a war as at best a misguided nuisance and at worst a liberal conspiracy to check American power.
Lord Goldsmith and Mr Blair had grown up with the cosier, more mainstream, European view which imbued the idea of international law with the spirit of a Wilsonian aspiration towards a world governed by rules, not might.
When Mr Blair, or his deputy John Prescott, got off the plane and found themselves in the embrace of the hardliners of the Bush administration, they found themselves not just in a foreign land geographically, but also in a foreign land intellectually.
It is very doubtful that any government lawyer would have ever advised Lord Goldsmith, as attorney general, that the invasion of Iraq without a second UN resolution could have been anything other than an illegal war of aggression. But suddenly, his boss found himself in an environment which was not just emotionally openly scornful of international law, but which backed up their scorn with arguments too.
They are arguments that fail, of course. Just because a lawmaking procedure is imperfect, does not mean that countries are allowed to pretend it is not there. And especially when that system of international law is endorsed by most of the rest of the world, and the world's sole superpower opts out only when it is to its advantage.
But the wider point is that exposure to the debate in the US changed the question in Mr Blair's mind from "would it be illegal?" to "does legality matter?" Which brings us back to the big week for the Chilcot Inquiry in front of which Tony Blair appears today.
I have, in the past few months, had some personal contact with Mr Blair. He taught a session at Yale which I attended, and it was interesting to see how he responded to the seismic shift in British public opinion on the legitimacy of the Iraq war. He hasn't given an inch.
He says, very openly, that he believes he made the right decision and insists that he would do the same thing again if he was in the same position. Even if he knew that Iraq did not have the weapons of mass destruction which were the nominal justification for the war in the first place.
He sees the invasion of Iraq as part of a wider, global, war of democratic, liberal countries, against the forces of terrorism and repression. This takes in his decision to invade both Iraq and Afghanistan.
Given this, I think that those questions to Mr Blair which focus on his rationale for the war will not provide the emotional payoff that his critics are looking for. He is not about to recant.
Where the inquiry can be the most fruitful, though, is in asking the former prime minister about how he managed to approve a war with such derisory postwar planning. Not even Mr Blair could assert that the years following the Iraq war went well. Many civilians were killed needlessly.
Chilcot has already learned how a senior civil servant called the swiftness of the defeat of the Iraqi Republican Guard a "catastrophic victory", as it left the forces completely unprepared for occupation. This is too polite. Had the victory been slower, the forces would still have been unprepared for occupation.
Chilcot should focus today on the allegation that Blair's acceptance of an invasion with such minimal post-war preparation cost lives. It is on that issue that Mr Blair's position is weakest.
Azeem Ibrahim is a research scholar in the International Security Programme at the Kennedy School of Government at Harvard University, a World Fellow at Yale University