Why did United states invade IRAQ?and how is their action justified.?

They used the world sympathy they got from the 9/11 event to slap Saddam with the accusation that he had and intended to use WMDs.

I would say that all dictators that have slaughtered people should be brought to justice. Saddam is hardly the only one, though... but no one else is being pursued... such as Assad or Kim.

That isn't right to single out one guy and let the others go.

Bush Jr. wanted the oil.

to defeat a dangerous enemy of the west and israel

to retrieve a 5,000-year-old weapon of mass destruction: the babylonian stargate device, hidden in the basement of the iraqi national museum.

do your own research and see for yourself.
saddam could have done more harm with it than if he'd had a nuclear weapon.
yes, it is real.