The military of the Han dynasty was the military apparatus of China from 202 BC to 220 AD, with a brief interregnum by the reign of Wang Mang and his Xin dynasty from 9 AD to 23 AD, followed by two years of civil war before the reestablishment of the Han dynasty. Han armies consisted primarily of infantry and cavalry, with minor use of chariots that became obsolete during the Western Han (202 BC – 9 AD). The Han naval forces included marines on tower ships built for inland riverine and lake battles rather than overseas ventures.

To the north along the eastern Eurasian Steppe, the Han dynasty fought against nomadic confederations such as the Xiongnu, Xianbei, and Wuhuan. To challenge these migratory peoples effectively, the Han Empire was compelled to develop, train, and supply large amounts of cavalry and mounted forces. The Han-Xiongnu Wars that began during the reign of Emperor Gaozu of Han reached a turning point during the reign of Emperor Wu of Han, whose forces pushed the rival Xiongnu westward with the conquest of the Gansu corridor and establishment of the Protectorate of the Western Regions over the Tarim Basin of Central Asia. Weapons such as the crossbow gave Han forces a slight advantage against armies that lacked them.

At the start of the Han dynasty, male commoners were liable for conscription starting from the age of 23 until the age of 56. The minimum age was lowered to 20 after 155 BC, briefly raised to 23 again during the reign of Emperor Zhao of Han (r. 87–74 BC), but returned to 20 afterwards. Some convicts could choose to commute their service by serving on the frontier. Conscripts trained for one year and then served for another year either on the frontier, in one of the provinces, or at the capital as guards. A relatively small minority of these conscripts would also have served in the cavalry division in the north, which was primarily composed of volunteers from families of superior status, or waterborne forces in the south. Conscripts were generally trained to arrange themselves in a formation five men deep, but actual practice on the battlefield could be flexible, with some commanders preferring ranks of up to 10 men deep. Implementation of daily best practices was also highly dependent on each commander, with some like Li Guang eschewing administrative details while Cheng Buzhi always kept his men in tight formations. After finishing their two years of service, the conscripts were discharged. During Western Han times, discharged conscripts could still be called up for training once a year, but this practice was discontinued after 30 AD.

Certain nobles were exempt from military conscription. Those of ranks four to eight did not have to perform service in their locality, and those of rank 9 and higher had full exemptions. During the Eastern Han period, commoners were allowed to commute military service by paying a scutage tax.

During the reign of Emperor Wu of Han, the standing army was significantly increased to face the military challenges presented by the Xiongnu during the Han–Xiongnu Wars and other opponents during the Southward expansion of the Han Dynasty. The number of cavalry forces was also significantly increased. The Han military by this time mostly favored longer-serving professional and semi-professional troops over troops that were regularly rotated and retired after a short length of service:

The total number of men eligible for the military draft was in the range of 13 to 15 million (corresponding to about 30 percent of the total population), and there was a standing army of 400,000 men in Wu-ti's early reign. Most important, as will be discussed later, at least 80,000 to 100,000 of the Han soldiers were cavalrymen-the future primary weapon of the Hsiung-nu campaigns and that number grew to 200,000 to 250,000 before 124 B.C. Afterward the total Han standing army would number 600,000 to 700,000 men, including about 300,000 cavalrymen, of which over 400,000 were professional soldiers, who had replaced the conscripts.

— Chun-shu Chang

Garrison troops and armies in the provinces and frontiers were often professional or semi-professional military colonists. These soldiers were stationed on the frontier and served in the military in return for a land allotment. This land was often in the frontier itself, and created a self-sustaining system where the soldiers and retired soldiers would be able to farm land that would produce the food that would feed their armies and local populace. The difference between military colonies and agricultural colonies was that the former provided regular military service as well as grain, whereas the latter only provided grain and/or taxes.