For over 50 years, the United States has pledged to help its European allies defend themselves against outside aggression under the Washington Treaty of 1949. The 1998 debate over enlarging the North Atlantic Treaty Organisation (NATO) to include three Central European democracies reignited the long-standing debate as to whether the United States bears an unfair share of the burden of the common defence. This new book examines several measures of defence burdensharing to determine how much the United States contributes to the collective defence relative to what other NATO allies contribute. It looks at traditional measures of defence budgets as well as military personnel and other metrics that measure countries' efforts to enhance collective security.