When students at Indonesia’s prestigious Institute of Agricultural Studies swore an oath to support a caliphate in the world’s largest Muslim-majority country last year, a video of the event went viral and the government grew alarmed.

Months later, Indonesian President Joko Widodo banned the decades-old hardline group Hizb ut-Tahrir Indonesia (HTI), which organized the student pledge, and declared its goal to set up a caliphate was incompatible with the constitution and could threaten security.

One former HTI member, Ayik Heriansyah, said the group tries to enlist support from influential members of society and sympathizers in the security forces to overthrow governments, or what it terms “the handing over of power”.

HTI has not disclosed its membership. Raymond Arifianto, a research fellow at Singapore’s S. Rajaratnam School of International Studies, said his sources say HTI had about one million members, including an estimated 10-15 percent of junior army officers as members or sympathizers.