BANGKOK—It’s a mystery that’s captivating Thailand: Who stole a brass plaque commemorating the revolution that ended royal rule 85 years ago?
As memorials go, it’s not showy. The 12-inch plaque was set in the roadway at Royal Plaza near a statue of King Rama V, a 19th century monarch. It marked the spot where in 1932 a group of military officers and civil servants declared the country a constitutional monarchy, relegating the king to a figurehead presiding over a series of elected governments and military juntas that endure to this day.
Days passed before anyone noticed the plaque was gone. Then, on April 14, word got out that it was stolen and replaced by a new plaque urging Thais to be loyal to their nation, family and monarch, King Maha Vajiralongkorn, or Rama X.
The swap made headlines, touching a nerve in a national debate over how this tropical Buddhist kingdom should be governed.
With pressure building at home and abroad, Thailand’s military rulers plan to hold elections next year after it took power in a coup nearly three years ago. But persistent questions remain over how much power Prime Minister Prayuth Chan-ocha’s junta is prepared to yield.
The military-appointed Senate will have a say who succeeds the general. The army will have the right to form a new government during times of crisis. And it’s unclear whether the junta will loosen rules that ban political gatherings of more than five people.
Diplomats and academics say the junta is trying to foment a nationalist mood in part to bolster support for Thailand’s royalist establishment and to further isolate former Thai leaders Thaksin Shinawatra and his sister, Yingluck Shinawatra, both of whom still enjoy a strong following in rural Thailand for their pro-poor policies.
Mr. Thaksin was ousted in a 2006 coup and lives in exile. A court removed Ms. Yingluck from office in May 2014 and she is now fighting graft charges for which she faces a lengthy potential sentence. She denies any wrongdoing.
“Thailand is entering a new era where the political divides will be sharper and more dangerous,” said Chotisak Onsoong, an activist who was once pelted with popcorn in a cinema for refusing to stand for the Thai royal anthem that is played before screenings.
Mr. Chotisak and other pro-democracy activists are trying to find out what happened to the plaque.
Some of them blame royalist-nationalists for the theft as part of a bid to recast Thailand’s history as devoid of any democratic tradition. Royalist groups have previously conducted ritual ceremonies by the plaque to pray for its divine disappearance. Vandals gouged its brass face.
One historian sympathetic to this view, Thepmontri Limpaphayorm, said he viewed plaque as an affront to Thailand’s monarchy and late last year issued a threat.
“If you don’t come dig it out by December 30, my friends and I will consider that there’s no owner,” he wrote on his Facebook page. “We will remove or destroy it ourselves. If you want to keep it as a souvenir, come and get it.”
Mr. Thepmontri, who once wrote a book criticizing a revolt against military rule titled “Peeling Back the October 14 Scab: A Worm on the Face of Thai History,” denies removing the plaque, but applauds its loss.
Police, meanwhile, say they don’t know who took it. Bangkok officials say security cameras in the busy area were offline for upgrades on the night the plaque was apparently removed.
Officials say they won’t pursue the case unless the owner steps forward to claim it—something the government hasn’t done. Gen. Prayuth has dismissed the plaque’s disappearance as a distraction.
“Please don’t make us solve issues that are not a matter of life and death,” he said, later warning demonstrators not to stage protests over it.
Still, when a constitutional expert attempted to present a petition to probe the theft at a government office, the response from armed soldiers suggested that the whodunit was more important than Gen. Prayuth suggested.
Before he could present his papers, troops intercepted Srisuwan Janya at the building entrance and took him off to any army camp where he was interrogated for 12 hours before being released late at night.
In an interview, Mr. Srisuwan said the soldiers offered him two meals, but that their goal was clear.
“They asked me to go slowly,” Mr. Srisuwan said. “The army doesn’t want to see any conflicts over this.”
Write to James Hookway at email@example.com