My belief is that they always knew there were never any WMDs there, and - yes - part of why they invaded Iraq was because of oil. You see, Saddam Hussein was threatening to quit the buying and selling of oil in the US Dollar, and wanted to use a different currency instead. This would have resulted in the US losing lots of money by having to pay more to import oil. Also, I am sure that there was a bit of revenge at play behind George Bush's motives to attack them because, years earlier, George Bush's father had failed in his attempt to overthrow Saddam.
George Bush Jnr. is arguably the most incomptent president the USA has ever had.