George Bush is seen in a picture during the Iraq War. 20 years have passed since the US invasion of Iraq.

The 20th anniversary of the U.S. invasion of Iraq is sparking much introspection. Some now see it as a grave error, others still think it was the right decision and others still who never supported the war now feel vindicated. With the benefit of 20 years of hindsight, three lessons from the invasion stand out.