The brigades were the main fighting units of the army. They were made up of five to ten regiments, which were called battalions when in battle. Some regiments were “Continental,” that is, organized and controlled by Congress, with long terms of service. Other regiments were “militia,” organized and controlled by the states, with short terms of service (usually one year). In addition to these standard regiments, when the Americans were doing well local militia companies suddenly showed up; they would depart whenever they pleased. Washington was infuriated at his lack of control over them. “They come in you cannot tell how, go, you cannot tell when; and act, you cannot tell where consume your provisions, exhaust your stores, and leave you at last in a critical moment.” He insisted that only a regular army with long terms of service could be properly trained, officered, moved from place to place, and counted upon to be around for a long campaign. http://en.citizendium.org/wiki/American_Revolution,_military_history