Answer based on context:

In January 2002, the United States Special Operations Command, Pacific deployed to the Philippines to advise and assist the Armed Forces of the Philippines in combating Filipino Islamist groups. The operations were mainly focused on removing the Abu Sayyaf group and Jemaah Islamiyah  from their stronghold on the island of Basilan. The second portion of the operation was conducted as a humanitarian program called "Operation Smiles". The goal of the program was to provide medical care and services to the region of Basilan as part of a "Hearts and Minds" program. Joint Special Operations Task Force - Philippines disbanded in June 2014, ending a successful 12-year mission. After JSOTF-P had disbanded, as late as November 2014, American forces continued to operate in the Philippines under the name "PACOM Augmentation Team", until February 24, 2015. By 2018, American operations within the Philippines against terrorist was renamed Operation Pacific Eagle, which involves as many as 300 advisers.

What group had their stronghold on the island of Basilan in January 2002?
Abu Sayyaf