No, there wasn't a specific event that triggered the 2003 invasion of Iraq and its legality is a matter of debate. According to (then) UN Secretary-General Kofi Annan the invasion was illegal:
When pressed on whether he viewed the invasion of Iraq as illegal, he said: "Yes, if you wish. I have indicated it was not in conformity with the UN charter from our point of view, from the charter point of view, it was illegal."
The main rationale for the invasion was that Iraq had Weapons of Mass Destruction (WMD). Iraq hadn't been particularly cooperative during the UNSCOM inspections (1991 - 1998) and in 1996 Scott Ritter alleged that the inspections had uncovered evidence of continuing chemical warfare research. Iraq responded by claiming Ritter was in fact a CIA agent, and that the inspections were used to cover up spying.
In 1998, Ritter was more or less convinced that Iraq had WMD and wouldn't allow the inspectors to find them. Bill Clinton signed the Iraq Liberation Act of 1998 into law and released a statement that reads as a prelude to war:
Now, let's imagine the future. What if he fails to comply, and we fail to act, or we take some ambiguous third route which gives him yet more opportunities to develop this program of weapons of mass destruction and continue to press for the release of the sanctions and continue to ignore the solemn commitments that he made?
Well, he will conclude that the international community has lost its will. He will then conclude that he can go right on and do more to rebuild an arsenal of devastating destruction.
And some day, some way, I guarantee you, he'll use the arsenal. And I think every one of you who's really worked on this for any length of time believes that, too.
Then, there was Operation Desert Fox, but a full scale invasion didn't happen. A few months earlier, Ritter resigned from UNSCOM, allegedly because he was dissatisfied that the UN wasn't taking harder measures against Iraq. Interestingly, in June 1999 he made the following statement:
When you ask the question, "Does Iraq possess militarily viable biological or chemical weapons?" the answer is "NO!" It is a resounding "NO". Can Iraq produce today chemical weapons on a meaningful scale? No! Can Iraq produce biological weapons on a meaningful scale? No! Ballistic missiles? No! It is "no" across the board. So from a qualitative standpoint, Iraq has been disarmed. Iraq today possesses no meaningful weapons of mass destruction capability.
A few months earlier, allegations that the US had indeed used the inspections to spy on Saddam surfaced. This time it wasn't the Iraqis doing the blaming, but Ritter himself. Later, he also accused the British for using the inspections as a cover up. The United Nations Security Council (UNSC) dissolved UNSCOM with Resolution 1284, in December 1999. The UNMOVIC took its place and a few months later Hans Blix becomes its Executive Chairman. The question why the UN didn't support/dictate/encourage a full scale invasion in 1998 remains unanswered.
Reports that Iraq was purchasing fissile material surfaced in 2001, and according to a declassified Donald Rumsfeld memo the focus was twofold: WMD and building momentum for regime change in Iraq. Interestingly, Rumsfeld puzzled about "How start?":
Allegations that the fissile material was yellowcake from Niger were deemed "highly doubtful" by Joseph Wilson following his investigation. Nevertheless, I think at this point in time the invasion had already been decided and it was only a matter of time. It's not just the aforementioned memo, Iraq was in a very tough spot in 2001, mainly because of the various theories (some of which presented compelling evidence) that connected Saddam's regime to Al-Qaeda. The world was slowly recovering from the September 11 attacks, and Saddam was again in the spotlight after a couple of relatively calm years.
To make matters worse, Iraq rejected several UN inspection proposals during 2001 and 2002. They finally decide to accept UNSC Resolution 1441 on November 13, 2002 and allow inspections to commence after four years. It could be argued that they came to their senses a little too late. The possibility of an invasion was already openly discussed, and potential military action against Iraq by the US had already been authorized. Months prior to the Iraq resolution Jack Abramoff alleged that an "upcoming war on Iraq" was mentioned to him by Karl Rove.
Regardless, the inspections started, and apparently without much resistance from the Iraqis. Hans Blix, in his presentation to UNSC on March 7, 2003, noted:
Inspections in Iraq resumed on the 27th of November 2002. In matters relating to process, notably prompt access to sites, we have faced relatively few difficulties, and certainly much less than those that were faced by UNSCOM [U.N. Special Commission] in the period 1991 to 1998. This may well be due to the strong outside pressure.
Some practical matters which were not settled by the talks Dr. [Mohamed] ElBaradei and I had with Iraqi side in Vienna prior to inspections or in Resolution 1441 have been resolved at meetings, which we have had in Baghdad.
Initial difficulties raised by the Iraqi side about helicopters and aerial surveillance planes operating in the "no-fly" zones were overcome.
This is not to say that the operation of inspections is free from frictions, but at this juncture we are able to perform professional, no-notice inspections all over Iraq and to increase aerial surveillance.
As I noted on the 14th of February, intelligence authorities have claimed that weapons of mass destruction are moved around Iraq by trucks, in particular that there are mobile production units for biological weapons. The Iraqi side states that such activities do not exist.
Several inspections have taken place at declared and undeclared sites in relation to mobile production facilities. Food-testing mobile laboratories and mobile workshops have been seen as well as large containers with seed-processing equipment. No evidence of proscribed activities have so far been found.
During inspections of declared or undeclared facilities, inspection teams have examined building structures for any possible underground facilities. In addition, ground-penetrating radar equipment was used in several specific locations. No underground facilities for chemical or biological production or storage were found so far.
The destruction undertaken constitutes a substantial measure of disarmament, indeed the first since the middle of the 1990s. We are not watching the breaking of toothpicks; lethal weapons are being destroyed.
One can hardly avoid the impression that after a period of somewhat reluctant cooperation, there's been an acceleration of initiatives from the Iraqi side since the end of January. This is welcome. But the value of these measures must be soberly judged by how many question marks they actually succeed in straightening out.
It is obvious that while the numerous initiatives which are now taken by the Iraqi side with a view to resolving some longstanding, open disarmament issues can be seen as active or even proactive, these initiatives three to four months into the new resolution cannot be said to constitute immediate cooperation. Nor do they necessarily cover all areas of relevance. They are, nevertheless, welcome. And UNMOVIC is responding to them in the hope of solving presently unresolved disarmament issues.
How much time would it take to resolve the key remaining disarmament tasks? While cooperation can -- cooperation can and is to be immediate, disarmament, and at any rate verification of it, cannot be instant. Even with a proactive Iraqi attitude induced by continued outside pressure, it will still take some time to verify sites and items, analyze documents, interview relevant persons and draw conclusions. It will not take years, nor weeks, but months.
All in all, Blix's report paints a rather positive picture for Iraq's stance towards this new round of inspections, at least in comparison with all previous rounds. Nevertheless, at this point almost everyone knew an invasion was imminent, and according to M. Tucker's and C. S. Faddis' book "Operation Hotel California: The Clandestine War Inside Iraq", the CIA was already operating within Iraq since July 2002. In a somewhat amusing incident, two days after Blix's presentation and eleven days before the conflict began Iraqi soldiers surrendered to British paratroopers of the 16 Air Assault Brigade in the Iraq/Kuwait border. A few days later, on March 16, the US abruptly advised UN inspectors to leave Iraq, and the next days the invasion began.
These are, more or less, the main events that lead to the war. I don't think there was anything sudden about it; this was a war years in the making. There's little doubt that the US, the UK and perhaps every other member of the coalition had very good reasons to want a regime change in Iraq. There's also little doubt that at the end the coalition forces chose to ignore the UN and go ahead with the invasion, regardless of the apparent shift in Iraq's attitude towards inspections.
As for WMD, I think everyone knows by now that nothing significant was ever found. Whether the coalition forces knew that Iraq didn't have WMD prior to the invasion is a matter of debate, but I wouldn't be surprised if they did and decided to go with the invasion anyway. Just for completion's sake, here's what Saddam himself had to say on the matter:
Saddam was also asked whether Iraq possessed weapons of mass destruction. "No, of course not," he replied, according to the official, "the U.S. dreamed them up itself to have a reason to go to war with us." The interrogator continued along this line, said the official, asking: "if you had no weapons of mass destruction then why not let the U.N. inspectors into your facilities?" Saddam's reply: "We didn't want them to go into the presidential areas and intrude on our privacy."