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. He became Chief of Staff of an Army that was stretched from 6 years of continuous war. Over his tenure he stabilized and transformed the army to meet the challenges of the 21st century while continuing to meet the demands of two wars. Casey transformed an army trained and prepared for conventional war, to an agile force more suited to modern challenges.