In his memoir Sihanouk Reminisces: World Leaders I Have Known (written by Bernard Krisher but dictated by the King, the man himself observes (p.70):
“… my small Cambodia was sandwiched between two powerful and threatening neighbors, Thailand and South Vietnam, which were both not only American allies but also in collusion with the Khmer Serei guerillas, whom they protected and trained. Both Thailand and South Vietnam also endorsed the “Anschluss’ of parts of Cambodia and they harbored these far rightist, anti-Sihanouk guerillas in order to further their territorial ambitions, which they justified with dubious interpretations of history.”
Based on what I have read of this book so far, I would classify Sihanouk as an unreliable narrator but, even so, it would be well for people to remember that there are longstanding reasons for discord between Thailand and Cambodia.
Incidentally, one of the reasons I love Asia Books is that occasionally little gems like this one will suddenly pop up – presumably, there are warehouses somewhere with old stock that occasionally are used to replenish the shelves.