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.
#2) Invading Iraq in the first place and staying there is just as big of a reason. The rest of the globe, including the middle east, hates us worse now than at any time in probably a 100 years. Europe is far different story than the middle east and you know it. Europeans are a secular group and are capable of rational thinking. The middle east is full of religious nut thugs that have any interest in EVER being peaceful. That region hasn't been stable ever in 600 years
#3)It's pretty simple. We can't trillion dollar deficit spend which would be required to keep our current military activism and a comprehensive energy policy. If we did the latter, the need for the first shrinks as well. It's silly to have a policy that makes us weaker to the terrorist (reliant on foreign oil) and makes them richer as oil continues to go up and up and up. I'd rather pay $15 billion a month for any other form of energy.
I'd also add that officer friends I know in the armed forces say the war was badly mismanaged until Patreaus came along. They're having to give very, very large signing bonuses to keep people on and are having trouble recruiting people. The Iraq war, while well intentioned, has been a complete disaster. Bush said the most the war would cost would be $50 billion. He fired his sec of treasury after he said it could cost $200 billion. Now it's over $700 billion and no end in sight. How much more of my tax dollars are you willing to waste on it?