The Constitution of the United States

The Congress shall have Power ... To declare War, grant Letters of Marque and Reprisal, and make Rules concerning Captures on Land and Water

Article II, Section 2 of the Constitution gives to the Executive Branch the command of the nation’s armed forces, while Article I, Section 8 gives to the Legislative Branch the power to decide when the United States goes to war. 

Congress attempted to shirk their duty to decide when the nation goes to war by passing the "AUTHORIZATION FOR USE OF MILITARY FORCE AGAINST IRAQ RESOLUTION OF 2002" 
 
On March 18th, 2003, President Bush submitted a letter of transmittal to the US Congress activating the Congressional Authorization for the use of force in Iraq. In that letter Bush stated that Iraq was in defiance of the United Nations' ban on weapons of mass destruction, and that the invasion of Iraq was needed to punish those responsible for 9-11. 

In hindsight, Bush lied on both points. Bush himself later admitted that Iraq had nothing to do with 9-11, and there never were any weapons of mass destruction. President Bush lied to the US Congress when he claimed to have proof of both of those assertions. The forged Niger "Yellow cake" documents, actions by the White House to keep the fact of that forgery secret, the "Dodgy Dossier", etc. all prove the lie was intentional with malice aforethought. That makes the first invasion of Iraq a war crime, illegal and unconstitutional, committed by the President, with Congress as accesories to that crime. 

The President is not allowed to lie to the people of the United States, certainly not to the Congress. The Congress, having been lied to, was duty bound to initiate impeachment proceedings at that point against President Bush. It was, after all, not sex for which Bill Clinton was impeached, but lying about it. Lying to Congress under any situation is a high crime or misdemeanor by any reasonable definition of those phrases. Lying to start a war is the worst crime a leader can commit against their own people. It is the highest of all High Crimes and Misdemeanors possible. 

So here we are with the people of Iraq taking back their nation (just as Iran did in 1979), and Obama is all set to order the US military forces to re-invade Iraq and teach those uppity people that the US now owns their nation and its oil. And once again, the President is starting a war without a formal declaration from Congress, and worse, Congress is refusing to obey their duty to call the President on his illegal and unconstitutional actions.
It was illegal for the US to invade Iraq in the first place, and equally illegal to invade it again!
 
As a side note, whenever a politician gets caught using government assets for private use, they are forced to reimburse the treasury. The same should hold true for the Presidents when they use public property, in this case the military, for private little wars of their own. Why should We The People pay for the costs of war (along with its ruinous interest) when the war is illegal? I think former President Bush should be billed for all of the material costs of the war(s) in Iraq. I know it is a lot of money, but I understand that the Bush family is fairly rich! :)