No! The invasion was not justified.
Tony Blair would like us to believe that the invasion came about because of concerns about Weapons of Mass Destruction (WMD). But that is a bare faced lie. The reality is that first Bush & Blair decided to invade & only after that decision was taken did they look around for reasons to justify the invasion.
George Bush was the more honest of the two, because he always admitted that toppling Saddam Hussein was the real reason for invasion. The problem with this approach is that it has no possible justification under international law. This means that the invasion was an "aggressive war" under the terms of the Nuremberg Conventions & was, therefore, a war crime.
Tony Blair relied more on WMD & argued that existing UN resolutions sanctioned the invasion. This was never true, although Blair did manage to find a couple of pet lawyers to argue that it was. However, all such arguments are rendered irrelevant by the way in which the war was fought.
Even a "just war" must be fought using "proportionate means", but "operation shock & awe" (the bombing campaign which killed tens of thousands of Iraqi civilians & destroyed the country's infrastructure) was disproportionate to any defence the Iraqis could possibly mount. So once again, the invasion has to be seen as a war crime.
To sum up:
(1) The invasion of Iraq was a classic example a "might is right" approach international diplomacy.
(2) Neither the invasion itself nor the way in which the war was fought can be justified under international law.
(3) Those responsible for the war, principally Bush & Blair, should be tried, convicted & executed for crimes against humanity.