Lieutenant Colonel Peter Dewey
 

Some might find it surprising that the first casualty of the Vietnam conflict was in September 26 1945. Lieutenant Colonel Dewey was part of the OSS Deer Team...
http://www.arlingtoncemetery.net/apdewey.htm

Given the date and circumstances I'd say that was a WW2 casualty ....

Interesting how in searches LTC Dewey comes up along side the team as well as the overlapping dates. He is not mentioned as part of that effort. It looks like there were several OSS projects going on in the transitional period of Japanese surrender.

This link goes a little deeper and reminds me of other missions at the time like Norway and the 99th Battalion Separate / later NORSOF and 474th Infantry Regiments although Indo China had different dynamics...
https://virginiareviewofasianstudies...d-Vietnam.docx