The Andorian Defense Force, or ADF, was an Andorian military organization which was founded in the 2160s following Andor's entry into the United Federation of Planets and was a successor to the former Andorian Imperial Guard. The ADF's main objective was to patrol and protect Andorian territories in conjunction with Starfleet vessels.
In order to serve in the ADF, Andorians had to attend the Andorian Academy where the cadets would undergo several years of vigorous training before they were allowed to serve on ADF vessels. (Star Trek: Away Team; Andorians: Amongst the Clans)
The ADF enjoyed a close partnership with Starfleet and the Federation, and would often participate in tracking down anti-Federation rebels in Andorian territory. In return, the ADF enjoyed Starfleet's technological support and the two forces worked together to design the Atlira-class in the early 2370s. (Andorians: Amongst the Clans)
During the Dominion War the ADF was assisted by the Starfleet Eighth Fleet for two-thirds of the war. Following the fall of Betazed they were further reinforced by the Twenty-Second and Thirtieth Fleets. (The Dominion War Sourcebook: The Fires of Armageddon)