The Zhou Dynasty began the enfeoffment system, stipulating that the armies of vassal states should be composed of more than 20,000 people, which is called an army (composed of three 7,500 people, each 7,500 is divided into three 500, each 500 is divided into ten 50 people, and each 500 is a team). So the largest is the army and the smallest is the team, which seems to be the origin of the name "army". )。

When Jin Wengong (the famous son Zhong Er) arrived, his national strength was very strong. He broke the ancestral system and expanded the army into three armies, with nearly 70 thousand people. He called these three armies the upper army, the middle army and the lower army. During the day, the countries had a fierce struggle to merge, so they followed Jin Wengong's example and expanded their armies into three, but with different names. For example, Chu was called Zuo Jun, Zhongjun and Youjun, and later it was called Qian Jun, Zhong Jun and Hou Jun. In the era of the former, middle and rear armed forces, due to the progress of the war, the tasks, functions and characteristics of the former, middle and rear armed forces were slightly different

In a word, this country has established the three armed forces for quite a long time, so it is customary to call all the armed forces the three armed forces, such as "rewarding the three armed forces" and "after doing the three armed forces".

The establishment of five people as friends, two as fire, five as fire brigade, two as officials, two as officials, two as songs, two as ministries, two as schools, two as schools and two as armies in the Han Dynasty is much better than the establishment of the three armed forces alone. But everyone is used to saying that.