NEFA Foundation did a study of Gitmo Detainees who were released and ended up back in the terrorism game:

• Mohamed Yusif Yaqub AKA Mullah Shazada; Yousef Muhammed Yaaqoub: After his release from GITMO on May 8, 2003, Shazada assumed control of Taliban operations in Southern Afghanistan. In this role, his activities reportedly included the organization and execution of a jailbreak in Kandahar, and a nearly successful capture of the border town of Spin Boldak. Shazada was killed on May 7, 2004 while fighting against US forces. At the time of his release, the US had no indication that he was a member of any terrorist organization or posed a risk to US or allied interests.
• Abdullah Mehsud: Mehsud was captured in northern Afghanistan in late 2001 and held until March of 2004. Mehsud said he spent two years at Guantanamo Bay after being captured in 2002 in Afghanistan fighting alongside the Taliban. At the time he was carrying a false Afghan identity card, and while in custody he maintained the fiction that he was an innocent Afghan tribesman. U.S. officials never realized he was a Pakistani with deep ties to militants in both countries. "I managed to keep my Pakistani identity hidden all these years," he told Gulf News in a recent interview. Since his return to Pakistan in March, Pakistani newspapers have written lengthy accounts of Mehsud's hair and looks, and the powerful appeal to militants of his fiery denunciations of the United States. "We would fight America and its allies," he said in one interview, "until the very end." After his release he went back to the fight, becoming a militant leader within the Mehsud tribe in southern Waziristan. We have since discovered that he has been associated with the Taliban since his teen years and has been described as an al-Qa'ida linked facilitator. In mid‐October 2004, Mehsud directed the kidnapping of two Chinese engineers in Pakistan. During rescue operations by Pakistani forces, a kidnapper shot one of the hostages. Five of the kidnappers were killed. Mehsud was not among them. In July 2007, Mehsud carried out a suicide bombing as Pakistani Police closed in on his position. Over 1,000 people are reported to have attended his funeral.
• Maulavi Abdul Ghaffar: After being captured in early 2002 and held at GITMO for eight months, Ghaffar reportedly became the Taliban's regional commander in Uruzgan and Helmand provinces, carrying out attacks on US and Afghan forces. On September 25, 2004, while planning an attack against Afghan police, Ghaffar and two of his men were killed in a raid by Afghan security forces.
• Mohammed Ismail: Ismail was released from GITMO in 2004. During a press interview after his release, he described the Americans, "they gave me a good time in Cuba. They were very nice to me, giving me English lessons." He concluded his interview saying he would have to find work once he finished visiting all his relatives. He was recaptured four months later in May 2004, participating in an attack on US forces near Kandahar. At the time of his recapture, Ismail carried a letter confirming his status as a Taliban member in good standing.
• Abdul Rahman Noor: Noor was released in July of 2003, and has since participated in fighting against US forces near Kandahar. After his release, Noor was identified as the person in an October 7, 2001 interview with al‐Jazeerah TV network as the “deputy defense minister of the Taliban.” In this interview, he described the defensive position of the mujahideen and claimed they had recently downed an airplane.
• Mohammed Nayim Farouq: After his release from US custody in July 2003, Farouq quickly renewed his association with Taliban and al‐Qa'ida members and has since become re‐involved in anti‐Coalition militant activity.
• Ruslan Anatolivich Odizhev: Killed by Russian forces June 2007, shot along with another man in Nalchik, the capital of the tiny North Caucasus republic of Kabardino‐Balkaria. Odizhev, born in 1973, was included in a report earlier this year by the New York‐based Human Rights Watch on the alleged abuse in Russia of seven former inmates of the Guantanamo Bay prison after Washington handed them back to Moscow in 2004.

DOD released a fact sheet in June 2008 about former Gitmo detainees:

• Ibrahim Shafir Sen was transferred to Turkey in November 2003. In January 2008, Sen was arrested in Van, Turkey, and charged as the leader of an active al-Qa'ida cell.
• Ibrahim Bin Shakaran
and Mohammed Bin Ahmad Mizouz were transferred to Morocco in July 2004. In September 2007, they were convicted for their post-release involvement in a terrorist network recruiting Moroccans to fight for Abu-Musab al-Zarqawi’s al-Qa'ida in Iraq (AQI). Recruits were to receive weapons and explosives training in Algeria from the Salafist Group for Preaching and Combat, which has since become al-Qa'ida in the Lands of the Maghreb, before going to fight in Iraq or returning to Morocco as sleeper cells. The organizers of the group reportedly intended to create an al-Qa'ida-affiliated network in the Maghreb similar to AQI. According to testimony presented at the trial, Bin Shakaran had already recruited other jihadists when Moroccan authorities broke up the plot in November 2005. Bin Shakaran received a 10-year sentence for his role in the plot, while Mizouz received a two-year sentence.
• Ravil Shafeyavich Gumarov
and Timur Ravilich Ishmurat were transferred to Russia in March 2004 and quickly released. Russian authorities arrested them in January 2005 for involvement in a gas line bombing. In May 2006 a Russian court convicted both, sentencing Gumarov to 13 years in prison and Ishmurat to 11 years.

CNN reported about a Kuwaiti terrorist who went right back to his old ways:

• Abdullah Saleh al-Ajmi was one of two Kuwaitis who took part in a suicide attack in Mosul on April 26, the officials said. Records show that an attack in Mosul that day targeted an Iraqi police patrol and left six people dead, including two police officers. U.S. military records of Guantanamo detainees indicate that a man with the same name and nationality was held at the Cuban prison. Those records said al-Ajmi, 29, was picked up in Afghanistan as he tried to enter Pakistan after the 2001 U.S. invasion. He claimed to have fought for the Taliban, the records show, and said he fought in a number of battles against the Northern Alliance. Though he was never charged with any crime, al-Ajmi was held at Guantanamo through 2005. Military documents show he later claimed that his statements about fighting for the Taliban were made after he was threatened while in U.S. custody. He asserted that he was in Afghanistan to study the Quran. Al-Ajmi was transferred to the custody of Kuwaiti authorities in November 2005, with four other Kuwaitis, and was released after a trial there, according to Pentagon officials.

Conclusion
The threat that we face in the 21st Century is nothing our forefathers could have envisioned. Unfortunately, politics have entered the fray to give unlawful enemy combatants rights that are typically reserved for U.S. citizens. Extending these rights and interfering in the conduct of military operations can be a very dangerous game that has come back to bite us.