United States Department of Defense Challenge Coin. This is a great looking United States Department of Defense challenge coin. Coin has reeded edges and is about 1 9/16" in diameter. All details are raised lettering and gold plated. Coin is contained in an acrylic airtight holder. The Department of Defense is an executive branch department of the federal government of the United States charged with coordinating and supervising all agencies and functions of the government concerned directly with national security and the United States armed forces. The Department is also the largest employer in the world, with more than 2.13 million active duty servicemen and women as well as their support staff of civilian workers. Adding to the total is over 1.1 million National Guardsmen and Reservists from the four services, bringing the total to just over 3.2 million employees.