Final answer:
Tony Blair was the Prime Minister of Great Britain during the 2003 invasion of Iraq, known as Operation Iraqi Freedom, which sought to remove Saddam Hussein under the unfounded premise of WMDs.
Step-by-step explanation:
The Prime Minister of Great Britain when Great Britain joined the United States in attacking Iraq and overthrowing the dictator Saddam Hussein was Tony Blair. This military action, known as the Iraq War or Operation Iraqi Freedom, commenced on March 19, 2003. The justification for the invasion was centered around the belief that Iraq possessed weapons of mass destruction (WMDs) and posed a significant threat to national security, claims that were later shown to be unfounded.