It should also be pointed out that no weapons of mass destruction were ever found in Iraq, and it became evident later that President George W. Bush only used this excuse to launch a second invasion that would ultimately end the reign of Saddam Hussein. Bush had long desired to stop Saddam, and his personal grudge against the Iraqi dictator--stemming from a bounty that Saddam had previously put on the head of Bush's father, the first President George H. W. Bush--was a prime motivation in his decision. According to testimony given by Richard Clarke, the former U. S. terrorism "czar," Bush was already planning for an invasion before the 9/11 terrorist attacks, and Bush's Secretary of State, General Colin Powell, said in 2001
... that sanctions against Iraq had prevented the development of any weapons of mass destruction by Saddam Hussein...
No evidence was uncovered that Saddam had any connection with the 9/11 attacks, nor were any WMDs ever found in Iraq. More than 6300 American troops have died since the invasion began in 2003.
The major cause of this war was the US belief that Iraq's leader, Saddam Hussein, had weapons of mass destruction and that he would be likely to use them or to allow them to be used against the US by terrorists. This was in the aftermath of the 9/11 attacks and there was a great deal of fear that Hussein would give terrorists the ability to carry out more destructive attacks.
There was also a great desire on the part of the Bush Administration to cause "regime change" in Iraq. President Bush and his advisors believed that Hussein was a major danger to the overall stability of the region.
Overall, then, the war was about the US fear of Saddam Hussein's ability to destabilize the region and/or attack the US with the weapons of mass destruction.