Domestic regime change is no stranger to Thailand, which has had at least 12 successful coups and 31 Prime Ministers since the founding of its current constitutional monarchy in 1932—itself the product of a coup. It’s for that reason that Prime Minister Paetongtarn Shinawatra, who took over after her fellow Pheu Thai party predecessor Srettha Thavisin was ousted from power last year, has sought to insulate the government against military takeovers, which have typically occurred in coordination with conservative, royalist elites. It’s likely, however, that she’s failed. Amid an escalating crisis sparked by border tensions with neighbor Cambodia, experts and...