He had questioned whether a duel on elephant-back, fought more than 400 years ago by a Thai king against a Burmese adversary, ever took place.
Sulak Sivaraksa was sent to a military court in Bangkok on Monday after reporting to a police station to hear whether the case against him, first lodged in 2014 by a military officer, would go ahead.
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The complaint was filed under Thailand's draconian lese majeste law, meant to protect top members of the royal family from defamation.
The law specifically excludes dead kings, but in practice the rules are often more widely interpreted.
