Weapons of mass destruction. None were found and the 'intelligence' that was used as evidence was shown to be anything but that. A classic case of the big lie. More to do with stabilising the M-E to protect western oil interests.
Where was the lie though? A lie is an intentional deceit.
This concept of an illegal war and lies justifying it have more to do with general misunderstanding as to public international law and actually law in general and how it works and also what intelligence is.
I feel I'm well qualified to say this as I am not only legally trained and qualified with a specialist in international law, but I am also a professional investigator.
I deal with intelligence and the NIM every day.
There is even sometimes a mistaken belief that intelligence = evidence, even amongst my peers, so it is no wonder the general public are confused.
Intelligence is not evidence. Intelligence is classified rumour, belief or suspicion.
Intelligence can be reliable or unreliable.
When you have to make a decision based on intelligence alone, you are always facing the possibility of your suspicions being incorrect. Lets be clear about this, there was no evidence of WMD's. There never was. What there WAS, was evidence of Saddam Hussein's intention to develop WMD's, his use of them in the past (nerve agents used on the Marsh Arabs), as well as his inclination to intimidate, attack and otherwise destabilise other nation states.
In addition to this there we the clear evidence of failure to co-operate with the cease fire terms of the Gulf War and mandated UN weapons inspections. This raises the "if you have nothing to hide..." factor. A bit like an innocent man saying "no comment" to every question in an interview under caution. It's not evidence of guilt, but it does raise the question of the motivation behind the behaviour.
In the context of 9/11 and the immediate aftermath, there is also an urgent need to prevent pariah states (let's not beat about the Bush here, Saddam's Iraq was just that, having breached countless treaty provisions and international conventions as well as being in numerous human rights violations.) having access to WMD's and enabling their access to terrorist organisations.
Out of date intelligence documents were used and an attempt was made to update them, but with limited sources_(given Iraq's non-compliance with UN resolutions, inspections and the nature of the country and Saddam regime).
The military capability of Saddam's regime was found to be well below the Gulf war capability and well below that suggested by intelligence sources. That applied not only to WMD's but to their conventional forces to which provided only a token resistance to the coalition invasion.
This happens all the time in law enforcement. Intelligence might be received that a major people smuggling operation is going through a local premises. Law enforcement raid the place and find no evidence of it. Or not enough evidence to do anything about it. Does that make the raid invalid?
In terms of the legality there was a UN security council resolution, valid at the time of the invasion, authorising use of force against Iraq.
Given the standard of evidence required the very fact you acknowledge there are valid arguments for the war being legal, means that it can't be illegal.
It's like a copper going to the CPS to charge someone with murder and saying, in my opinion he killed the bloke, although I acknowledge that DC Plod over there makes valid points as to how he is probably innocent.