He served as Commanding General, Multi-National Force – Iraq from June 2004 to 8 February 2007, and was in the army for his entire adult working life.

He was killed on 7 July 1970, when his command helicopter crashed in South Vietnam en route to a hospital to visit wounded American soldiers.

Following these assignments, Casey was nominated and confirmed as the 30th Vice Chief of Staff of the Army, serving in that post until June 2004.

He expressed his view that a large and intrusive American presence in Iraq would not solve the political and security problems in that country and could even fuel the insurgency.

The bombing of the al-Askari Mosque, a sacred Shia religious site in Samarra, is believed to have stoked sectarian tensions and derailed coalition plans to speedily transfer significant security responsibility to the Iraqi government by the end of 2006.

It's always been my view that a heavy and sustained American military presence was not going to solve the problems in Iraq over the long term.

Casey officially succeeded General Peter Schoomaker as Chief of Staff of the Army on 10 April 2007.

As the 36th Chief of Staff of the United States Army from April 2007 to 2011, Casey led what is arguably the world's largest and most complex organization—1.1 million people strong, with a $200+ billion annual budget—during one of the most extraordinary periods in military and global political history.

Casey transformed an army trained and prepared for conventional war, to an agile force more suited to modern challenges.

[citation needed] In the immediate aftermath of the 2009 Fort Hood shooting committed by United States Army psychiatrist Nidal Hasan, Casey expressed concern about jumping to conclusions before the investigation was completed, telling CNN's John King that "this increased speculation could cause a backlash against some of our Muslim soldiers" and "As great a tragedy as this was, it would be a shame if our diversity became a casualty as well.