1950s Cambodian soldier

The caption states from 1950 but may be later.
Certainly a heavy US influence already.
Possibly British gaiters worn with jungle boots.
Again from Getty Images

https://i.servimg.com/u/f72/12/22/09/10/50camb10.jpg
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No doubt the uniform and equipment he's wearing were items we'd given the French in WWII and were left behind by them when they pulled out of Indochina after the 1954 Geneva Agreement.

That also looks like a Thompson SMG that he is carrying.

Certainly is... and I think the guys in the background have a Garand rifle and a 1911 Colt pistol

British KD uniform with 1937 web ankets, US M1 helmet.