1) One of the things we learned from the Iraq war was that Iraq had missles that were in violation of the terms of the cease fire. These missles had a range that was well beyond what was allowed. We also found several tons on uranium, instructions to manufacture biological agents and many tons of chemical weapon precursors. We would not have been safe if we had left Iraq to operate as usual.
2) To invade Iraq and then simply leave it burning would only give militant Islam a reason to attack us again. By changing Iraq, we are changing the opinion of the Muslim world. Al Qeada has gone from being the savior of Islam to its enemy. This is significant. Also, if we take our lessons from WWI and WWII, it is shown that punishing a beligerant country (post WWI Germany) only creates bigger wars, while repairing a country (Marshall Plan, post-WWII Japan) has benefits that far far exceed the costs associated with the repairs.
3) You will have to explain to me how we need the money that is being spent in Iraq to pursue a domestic energy policy. There are at present over two dozen domestic nuke plants that are in one stage of planning or another, being built by the private sector...while we fight in Iraq. ANWR/Gulf of Mexico/California oil exploration is stalled due to environmentalism and no amount of money will get it moving.