If a war is to be considered just, then it must be declared by legitimate authority (ST II-II, q. 40, a. 1c; CCC 2309).
In the United States of America the authority to declare war is given to Congress (Constitution of the United States, article I, section 8).
But the current war in Iraq was not declared by Congress.
Therefore the current war in Iraq was not declared by legitimate authority; and from this conclusion it follows that the current war in Iraq is not a just war.