ANTWERP, Belgium—Nabil Kasmi left for Syria's battlefields in May 2012, the first jihadist an extremist Islamic group named Sharia4Belgium dispatched from this city, Belgian authorities allege.

On Monday, the 23-year-old Mr. Kasmi and 45 other Sharia4Belgium members will go on trial, in Europe's most high-profile legal effort yet to address a dangerous new reality of the bloody war in Syria and Iraq: Europe is increasingly becoming a recruiting ground for jihadists heading there to join terrorist groups like Islamic State.

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