After his daughter was sidelined from political office by a damaging phone call with Cambodia’s leader this month, the Thai power broker Thaksin Shinawatra broke his silence to a roomful of politicians and journalists. He had a stunning message to deliver: His decades-long relationship with the Cambodian strongman, Hun Sen, was over.

“I used to be close to him — like brothers,” Mr. Thaksin said. “But after what he did to my daughter, I was shocked. How could this even happen?”

For years, many had believed that the personal relationship between Mr. Thaksin, 75, and Mr. Hun Sen, 72, would be the glue holding the two neighbors together despite an intractable border dispute. Both were among Southeast Asia’s most seasoned politicians, bound by a friendship spanning 33 years and by their shared dynastic ambitions, with children who were elevated to power within a year of each other.

Now, a rift has opened up between the two men, bewildering even Mr. Thaksin himself and shocking insiders. And the fallout has been severe, with Thai and Cambodian troops exchanging fire in the deadliest clashes in over a decade. Analysts say they worry that the animosities could spiral out of control, driving the region into a broader war.

“I was surprised how two close friends for so many years ended up practically overnight in such an escalation downward,” said Kantathi Suphamongkhon, who was Thailand’s foreign minister from 2005 to 2006 when Mr. Thaksin was premier. “This is something that I never expected — how that friendship can break apart so spectacularly.”