The War in Iraq was a poor idea. Here are the principle reasons why the War in Iraq was (and is) a bad idea.
- No justification for invasion
- Casualties of Iraq War
- Almost $3 trillion in financial costs
- No plan after invasion
- Damage to U.S. alliances over world
- Disruption in Oil production
C1: No justification for invasion
There was no good reason for invading Iraq. No weapons of mass destruction were ever discovered after the invasion of Iraq. Therefore, Bush's statement that we have to eliminate their weapons of mass destruction is false. There were NO weapons of mass destruction in Iraq.  Plus, it is widely argued that we illegally invaded Iraq, violating international law. 
C2: Casualties of Iraq War
Because of the Iraq War, over 4,100 Americans have lost their lives, believing that they were protecting their country. Why should they sacrifice their lives for an unjust cause?  Additionally to this, over 150,000 Iraqis have lost their lives, and over 80% of these deaths were civilians.  Also, now half of Iraq's children are orphans. 
C3: Almost $3 Trillion in financial costs
This is outrageous. $3 trillion in costs for no real reason, it is not a good idea to use taxpayers money to kill hundreds of thousands and dismantling a country and endangering a region. 
C4: No Plan After Invasion
We had no plan after invasion. All the focus was to get Saddam Hussein, but then what? Did Bush say let's make Iraq a democracy and nation build! If you did hear this, I'd like to see proof.
C5: Damage to U.S. Alliances over world
The USA got a negative reputation worldwide after unilaterally acting on invasion for no real reason. With a few exceptions, many nations in the middle east have been angered at our actions, including traditionally important allies such as Israel. 
C6: Disruption in Oil Production
Since the Iraqi War began, oil prices have been volatile, and the $3 trillion could of been invested into domestic renewable energy to save tens of thousands of lives, reduce the effects of global warming, save much money, and have numerous other benefits, and a cleaner environment. Still, oil prices have tripled since the Iraq War began.