Thailand and Cambodia signed an expanded ceasefire agreement on Sunday during a ceremony attended by U.S. President Donald Trump.

When an estranged son of Thailand’s King Maha Vajiralongkorn turned up in the capital Bangkok earlier this month after a three decade-long absence, it caught many in the kingdom by surprise.

Following a border conflict that killed dozens of people and displaced hundreds of thousands more, Thailand and Cambodia agreed to an “immediate and unconditional ceasefire” on Monday, The Washington ...