anti-chemical (nuclear, biological and chemical defense), communications, special operation, logistics, reconnaissance, electronic warfare, etc. These branches of service have their respective military education institutions. Operational structure PLA operational structure reflects China's strategic missions, political environment, and geographical circumstances. There are 13 corps sized group armies (, also known as combined corps) since the end of April 2017, divided among five Theater commands — Eastern, Southern, Northern, Western and Central. Within the group armies, the old divisions () are being downsized into brigades (). Each group army includes six maneuver combined arms brigades, fire support/artillery brigades, air defense brigades, aviation brigades, special operations brigades, combat support brigades, and sustainment brigades. The maneuver combat components of the group armies are combined arms brigades (), including a mix of heavy combined arms brigades, medium combined arms brigades, light combined arms brigade, amphibious combined arms brigades, and mountain combined arms brigades. The practice is functionally similar to the US Army brigade combat team concept with unique modifications influenced by China's terrain diversity, strategic priority, political system, and military history. The PLA heavy, medium, and light combined arms brigades share a modular construct, resembling its superior and subordinate units at the corps and battalion level. A typical PLAGF combined arms brigade has the brigade HQ, four maneuver combat battalions, and other support battalions. For instance, a heavy combined arms brigade includes four combined arms battalions (), one artillery battalion, one air defense battalion, one reconnaissance battalion, one combat support battalion, and one sustainment battalion. Combined arms battalions apply a structure drawn from the brigade echelon. For example, heavy combined arms battalions consist of battalions HQ company (including subordinate medic, reconnaissance, and air defense platoons), four maneuver combat companies including two tank companies (14 tanks per company), and two mechanized infantry companies (14 vehicles per company), one firepower company, and one combat support/sustainment company. Before the 2015 reform, the International Institute for Strategic Studies (IISS) attributes the PLA Ground Force with nine active armored divisions consisting of a number of armored brigades, 25 infantry divisions (mechanized or motorized), organized into a number of infantry brigades, and 8 artillery divisions, also organized into field artillery brigades. Dennis Blasko wrote in 2000 that the traditional structure of PLA divisions (armored and mechanized) consisted roughly of three regiments – tuan () – of the main service arm, each of three battalions () plus support units, a fourth regiment/brigade of