Economic growth in the United States, as measured by Gross Domestic Product (GDP), has varied over the past 50 years and has impacted federal spending, as well as defense and non-defense spending as a share of GDP. Nevertheless, as more government programs were developed and expanded (such as Social Security and Medicare), spending in those areas steadily increased as a share of GDP. Military expenditures, on the other hand, despite procurement and development of new types of equipment, as well as expenses from operations and maintenance and the rising costs of military healthcare and retirement, declined as a share of GDP. Within the context of this overall trend, military expenditures fluctuated over various periods (the Korean War, the Vietnam War, the Cold War, and the War on Terror) due to the assessment of global challenges and opportunities, shifting defense priorities, and the types of equipment and manpower that were necessary.