The New York Times has a nice overview of the US Special Forces Command (USSOC).
USSOC was established in 1987 and is in charge of small, elite units with about 54,000 active duty personnel across all four branches of the armed services. Its counterterrorism units were responsible for killing Osama bin Laden and capturing Saddam Hussein.
USSOC has grown in size and importance since September 11th, as the military focused on the war on terror. Donald Rumsfeld, then Secretary of Defense, wanted an aggressive and unilateral force that could carry out the dirty and dangerous jobs. In 2004, he authorized secret offenses in more than a dozen countries, with SO troops carrying out the mandate in Syria, Pakistan, and Somalia. Seen as a bakdoor for the Pentagon to carry out offenses in non-combat zones, the command stepped back from the plan in May 2008.
The Joint Special Operations Command (JSOC) is an even more clandestine element of USSOC. Lt. Gen. Stanley A. McChrystal, who in May 2009 was chosen to become the new top American commander in Afghanistan, a forner commander of JSOC ‘pushed hard for a secret joint operation in the tribal region of Pakistan in 2005 aimed at capturing or killing Ayman al-Zawahri, Osama bin Laden’s deputy, which was cancelled at the last minute.
In May 2010, The New York Times reported that Gen. David H. Petraeus in September 2009 had ordered a broad expansion of clandestine military activity in an effort to disrupt militant groups or counter threats in Iran, Saudi Arabia, Somalia and other countries in the region.The secret directive authorizes the sending of American Special Operations troops to both friendly and hostile nations in the Middle East, Central Asia and the Horn of Africa.