For a picture with little detail you really got the details perfect, especially his boots and how defused he looks in that mist!
Was this based on a photo by any chance?

No, but form itself based on WW1 military form, with some differences (something like parallel universe )

The idea is a uniform based mostly on WW1 and WW2 Marine Corps uniforms. With a little hints of modern design.
Story is the species developed anthropomorphic features through evolution on another planet. Earth was fled from because of a terrorist disaster that couldn't be controlled in time so each nation had to board arch ships to travel into space. One of the ships discovered the planet of Dealron. Which they tried to colonize but due to losing a lot of important technology they didn't have the medical power to deal with the other worldly viruses on this new land. Most the animals they brought were discovered to be infected so.. they were abandoned.
The old ruins are now considered relics to the animals. But most of it was destroyed by time. So they lived their own time-line very similar to humans. One of the recovered relics was a "US Marine Corps Handbook" which was later seen as kind of like a Bible from their human gods. They have never met humans but are aware of them. The handbook was also heavily damaged so the uniform is an imperfect version of the US Marine uniforms.

Nicely Done!
I hope this gets expanded on in the future cause it sounds like a fascinating story!

Expertly done! Very nice!