Why didn't Canada join the United States when it invaded Iraq?

The USA made the case that Saddam Hussein was building weapons of mass destruction (chemical and nuclear)

U.S. President George Bush also made the claim that Iraq had a connection to the terrorist attacks that took out the twin Towers in New York on September 11th, 2001. He threatened that despite the fact that  U.N. Officials were not able to find evidence of WMDs. Saddam Hussein needed to be taken out of the picture. Ironically Saddam Hussein originally came to power in Iraq through U.S. influence over the country.
See the Wiki on Saddam Hussein here

Canada however refused to join the United States in its mission in Iraq citing the lack of support from the United Nations. (you only need to watch the first two minutes of this video)

In the end George Bush admitted that the main reasons that the U.S. gave for invading Iraq were largely false.

Some people suggest that former President Bush should be held responsible for war crimes under the Geneva Convention. This is especially due to how heavily the Bush administration used torture to extract information from many of the people it captured in the war in Iraq.

Even as we see in this article the use of waterboarding (which admittedly was used under Bush's administration), has been condemned by members of the Republican party.

What do you think were the real reasons why the USA invaded Iraq? How does this relate to its national interests ( economic prosperity, security)? How does this situation mirror the way balance of power has been managed in this region since WW1?