The government has outlawed the Islamic State group and spoken forcefully against it, as have mainstream religious groups in the world's largest Muslim nation. One fear is that militants who travel abroad will return home and conduct terrorist acts in Indonesia.
The West Jakarta District Court began separate trials for Ahmad Junaedi, Ridwan Sungkar, Helmi Muhammad Alamudi and Abdul Hakim, as well as two others who helped them go to Syria to join Islamic State jihadists.
The seven men being tried, aged 32 to 51, were arrested in police raids in late March and early April in the capital, Jakarta, and East Java's Malang town.
State prosecutor Anita Dewayani told the court that Junaedi, Sungkar, Alamudi and Hakim attended jihadist training organised by IS in Syria, pledged allegiance to the group's leader Abu Bakar Al Baghdadi, and fought with the group for up to seven months.
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If found guilty, the seven defendants would face up to 20 years in prison under Indonesia's anti-terror law.
Indonesian authorities estimate over 600 Indonesians have joined the ISIS Daesh in Syria or Iraq.
For the first time since the 1990s and the Afghan jihad, Indonesians, Malaysians and other extremists in Southeast Asia are travelling abroad in an organised fashion to join a global militant movement, picking up battlefield skills and militant contacts.
Radicals at home also could heed the Islamic State group's exhortations to carry out revenge attacks on Western targets.