Ihe IDF includes fighters who have been recruited since they were in high school and trained to the highest standards - the special forces. The road to becoming a member of the special forces is long and grueling. Each candidate must pass a selection process that usually lasts 3 to 5 days. The soldiers undergo activities meant to test their reaction under conditions of sleep deprivation, physical exhaustion and extreme mental stress. Hundreds will start the selection process, but only 50 to 100 usually complete it. And only the best 25 or so are chosen to enter the special forces unit. The soldiers then partake in a training program that usually lasts about 20 months. Unlike special forces personnel throughout the world, who work their way into elite units, Israeli special forces soldiers begin their careers in the elite units. If they are unable to keep up, they are transferred to less demanding special forces units or to conventional combat units. Israel's special forces can be traced back to before the establishment of the state. In 1941, the Hagana created the Palmach strike force, which is generally regarded as the first Israeli special forces unit. Since then, many special forces units have been established to fight on the ground, air and sea. There are more than 20 special forces units, each with its unique jobs and qualifications. In the ground forces, there are special forces units in the infantry, Armored, Combat Engineers and Field Intelligence Corps. There are also a number of special missions units, such as the Sayeret Matkal, the General Staff Reconnaissance Unit. These units perform a number of missions ranging from gathering intelligence behind enemy lines, to clearing paths and providing demolition support in enemy territory for the rest of their battalion. The IAF and the Israel Navy have special forces as well. The IAF, for example, has an Airborne Rescue and Evacuation unit whose mission is top retrieve downed personnel deep within enemy territory. There is also the IAF's secretive Shaldag battalion, which is involved in target acquisitioning. The navy's special forces include Shayetet 13, the Naval Commandos. Despite their superb capabilities, the special forces do have problems. One difficulty is that due to the multiplication of such units, many have overlapping responsibilities and expertise. In addition, because mandatory military service lasts only three years, by the time the soldiers complete their training, they sometimes have just over a year left in the army, creating a constant need for replacements.