chiefs serve for four years in one four-year term. The Chief of the National Guard Bureau serves a nominal four years. Extensions of the standard tour length can be approved, within statutory limits, by their respective service secretaries, the Secretary of Defense, the President, or Congress but these are rare, as they block other officers from being promoted. Some statutory limits can be waived in times of national emergency or war. Retirement Other than voluntary retirement, statute sets a number of mandates for retirement. A general must retire after 40 years of service unless they are reappointed to serve longer. Otherwise all general officers must retire the month after their 64th birthday. However, the Secretary of Defense can defer a general's retirement until the officer's 66th birthday and the President can defer it until the officer's 68th birthday. To retire at four-star grade, an officer must accumulate at least three years of satisfactory active duty service in that grade, as certified by the Secretary of Defense. History and origins See also List of active duty United States four-star officers List of United States Army four-star generals List of United States Marine Corps four-star generals List of United States Air Force four-star generals List of United States Space Force four-star generals List of United States military leaders by rank Staff (military) Title 10 of the United States Code United States Army officer rank insignia United States Marine Corps officer rank insignia United States Air Force officer rank insignia References Military ranks of the United States Army Military ranks of the United States Marine Corps Officer ranks of the United States Air Force Officer ranks of the United States Space Force Four-star officers