There is no argument, at all, for declaring the Iraq War unjust when Iraq BROKE the ceasefire from the first gulf war.
How many agreements have Iran and North Korea violated? There are countries all over the globe that violate agreements and treaties and sanctions and human rights, I don't see us invading them.
Just because we can invade a country doesn't mean we should. We should have stayed in Afghanistan, destroyed AQ and the Taliban, and captured or killed Osama bin Laden and his lieutenants as a response to 9/11, not veered completely off course in the name of 9/11 into a country that had nothing to do with it, taking our eye off the ball, allowing bin Laden to escape, and making the US less safe by stretching our forces thin and overthrowing a stable secular regime (however brutal it may have been), creating a power vacuum and turning Iraq into a terrorist hotbed, drawing insurgents from Pakistan and Iran to Iraq for the opportunity to wage war against American military forces.
We went in with no understanding of Iraq's politicial, social, or religious makeup, claiming an atheist like Hussein who headed a secular government would be allied with religious fanatics like AQ who hate secular governments like his (and ours) and want to institute fundamentalist Islamic theocracies, trying to impose democracy on a country that has no tradition of it, and completely underestimating the thousand-year-old religious and ethnic divisions between different groups that Saddam Hussein had (albeit by brutal methods) temporarily had under control.
Our justification for attacking Iraq was based on lies and faulty or manipulated intel, based on a nonexistent 9/11 connection and WMDs that weren't there, and have resulted in embroiling us in a military and political quagmire that is costing us thousands of lives, God knows how many Iraqi civilians, billions upon billions of dollars when we are in the middle of the worst economic recession in decades, and is diverting resources, manpower, and attention away from the REAL frontline in the war on terror and the REAL source of 9/11, Afghanistan....or at least Afghanistan was the frontline in the war on terror before we went and created a second front through our own unnecessary actions in Iraq.
This Iraqi war was a colossal screw-up from the beginning and continues to be a massive drain on lives, resources, and money that we are in no position to afford.