The Imperial Starfleet Military (also known as the ISM) is a large Federation Alliance military organization designed to supplement the Federation Starfleet with raw military power. In a crisis, Federation officers will take command of an ISM legion and use it to end a threat. The ISM is an organization separate but subservient to the Federation Starfleet. The ISM was formed in early 2385 due to the new Borg offensive. This group was based on the marines of naval vessels of old. (Star Trek: Unity (fan film series))
As part of the Alliance armed forces, ISM armies are made up of troops and guards from Alliance aligned non-Starfleet military organizations such as the Andorian Guard and the Military Assault Command of Earth.