by clandestine means, such as parachuting at night (especially using HAHO or HALO techniques), delivery by naval special operations vessels or from submarines, by out-of-uniform infiltration from a neighboring country, sub-surface infiltration utilizing closed circuit dive gear (combat diver), etc. If the infiltrating party is to be met by local supporters, there must be pre-agreed recognition signals. Should the infiltrators not be able to find their local contacts, they should have a variety of backup plans, ranging from establishing a clandestine base and waiting for contact, or to be recovered by their own side. Early in an insurgency, electronic communications should be avoided, as enemy SIGINT might learn of activities simply by detecting an unexpected radio signal. Couriers and personal meetings are resistant to SIGINT, but also have problems of maintaining physical security. Organization Citizen soldiers of the guerrilla force, underground and auxiliary are trained and equipped for their intended roles. SF personnel, possibly supplemented with communications and security experts in the AO, as well as support organizations outside the country, create the clandestine cell system to be used by hidden units. In this phase and later phases, SF medical personnel often work to improve the health of their sympathizers. Buildup The operation increases recruiting, and may begin clandestine intelligence collection and subversion, and possibly some hit-and-run raids and ambushes that have a high probability of success and a low risk of compromising security. Another covert operational technique, which may be used during this phase, is placing improvised explosive devices (i.e., mines and boobytraps). Sabotage, such as placing an explosive charge under a railroad track, is easier to hide than a raid on the train. If there is aerial bombing of targets, it can be used as a cover for sabotage activities. Forces that have reached this level have significant intelligence collection, sabotage and subversion, and fire direction capabilities. Employment In the combat phase, the guerrilla force increases the tempo of operations, in a manner consistent with its own safety and security, until the government falls or the guerrilla force links up with conventional forces. See raids and ambushes for the classic guerrilla combat operations, which may not be appropriate for the changing operational environment. Transition A stable country will no longer have autonomous guerrillas. The guerrilla force may form the nucleus of a new military, come under the control of the new national government, or go back to civilian