The Regular Army of the United States succeeded the Continental Army as the country's permanent, professional land-based military force. Even in modern times the professional core of the United States Army continues to be called the Regular Army. From the time of the American Revolution until after the Spanish–American War, state militias and volunteer regiments organized by the states (but thereafter controlled by federal authorities and federal generals in time of war) supported the smaller Regular Army of the United States. These volunteer regiments came to be called United States Volunteers (USV) in contrast to the Regular United States Army (USA). During the American Civil War, about 97 percent of the Union Army was United States Volunteers.