Dream of Becoming a Navy Seal or Part of Delta Force? Here is the Ultimate Guide to U.S. Special Forces.

The United State's Special Operation Command (SOCOM) counts over 70,000 personnel drawn from elite units in the Air Force, Army, Marine Corps and Navy. At times it virtually resembles a fifth service of the U.S. military with its own aircraft, boats and support services.

Special operations forces are employed for high-risk missions where use of blunt conventional firepower is constrained by political or tactical factors, and where finesse and discretion are required. That includes 'direct action' missions such as hostage rescue and capture or assassination of critical enemy personnel, as well as communicating with and training regional allies, and performing reconnaissance deep inside hostile territory.

Members of special operations undergo extreme tests of physical and mental endurance as part of their training. Culturally, special ops units place greater emphasis on smarts and individual initiative, and eschew being labeled 'soldiers', instead calling themselves 'operators' or unit-specific titles like Raider, Ranger or SEAL.

In this first part of a two-part series, we'll look at the overarching structure of U.S. special forces, and the diverse special operations units fielded by the U.S. Army.

U.S. special ops units fall broadly into two categories. Tier 2 and 3 units are usually assigned to service- or region-specific commands, operating only under the auspices of SOCOM when coordinating with other special forces units.

Meanwhile, Tier 1 units, also known as Special Missions Units, are directly commanded by the national-level Joint Special Operations Command (JSOC), which undertakes critical, classified missions approved at the highest level. For example, JSOC has organized task forces in Iraq and Afghanistan to hunt down senior leaders of Al Qaeda, ISIS and the Taliban.

Known Tier 1 units include the U.S. Army's Delta Force, Ranger Regimental Reconnaissance Company, and Intelligence Support Activity; the Navy's DEVGRU (SEAL Team 6); and the Air Force's 24 ^th Special Tactics Squadron. JSOC occasionally also integrates lower-tier units when necessary to fulfill operational requirements.

Special ops units also often work closely with U.S. civilian intelligence agencies, particularly the CIA's shadowy Special Activities Division, which draws many of its personnel from the Special Ops community.