I was opposed to the Iraq invasion when it occurred. Bush Jr. clearly had a hard-on to be a "war president" like his pappy. The reasons for the invasion didn't matter. U.S. troops began mobilizing before fabricated evidence for WMD's was presented to the U.N.. When the U.N. wouldn't give the U.S. the mandate to invade, the U.S. went ahead anyways and bullied/bribed other countries to join their coalition to lend it legitimacy. Canada had no choice about invading Afghanistan, but they had the option to stay out of Iraq and it was the right decision to do so.
The invasion may have been over fairly quickly, but the U.S. really bungled the occupation. They never established firm control over all parts of the country. Then they cut and ran, leaving behind a woefully unprepared Iraqi army to try to hold the country together. ISIL stepped into the power vacuum and are now committing atrocities that make Saddam's rule look like playtime with Howdy Doody by comparison. Slavery, child rape, video executions, destruction of archaeological sites, mass murder, you name it. ISIL is deliberately savage.
ISIL is a monster created by the U.S., so the U.S. bears the responsibility of dealing with it. Reigning in ISIS and establishing order where barbarians are running amok is quite a different thing from invading a sovereign nation. ISIL is expanding and threatening populations that want nothing to do with them. Letting ISIL invade and brutalize town after town is, morally, abominable. The U.S. is doing he right thing by going back to help clean up the mess they created. Canada is doing the right thing by helping. It wasn't our mess but we can help clean it up.
What we should pressure our leaders to do is come up with a viable exit strategy. We can't just drop bombs and hope for the best. We need a plan that destroys ISIL and leaves a competent power in their place. The U.S. tore the Iraqi nation down. Now it's time to build it back up, unless we're happy with ISIL spreading throughout all of Iraq and likely significant parts of Syria.
It's not Canada's responsibility, but destroying ISIL needs to be done.