I want to use this french military song in a commercial project. This one was done for the waterloo movie (1970) https://www.youtube.com/watch?v=HXmljXjwZko , anyone knows where I can acquire a license for this track?

You want a license for the composition? Or for the performance?

@Tom Sloper The performance. Not specifically this performance though. I want to be able to use this song in a commercial project. I'm trying to find the license of this performance but there's very little info about it except that it was made for the 1970 movie which I believe would still hold the copyrights to the soundtrack.

If you want that performance: Get a copy of the movie and find the details about the performance in the movie's credits. Or maybe the music score credits are listed on imdb.

I can't tell you where to get a license for that track, just wanted to chime in that licensing is a pain :(. I found a song that fit perfectly for a trailer I was making. After tracking down the right people, and emailing back and forth providing them all kinds of info on what I was making, how I'd be using it etc., they offered me 6 months limited rights for $2k. From now on, I'm either going cc0 music, or I'll get some custom made on fiverr for cheap.

A bit more info here:

Given that this is a period piece, it might be public domain. I don't know the French equivalent but in the US works of a certain age can be use publicly without issues. This can get pretty complicated however because it doesn't extend to recordings just the composition. And estates or businesses can always opt to extend out a copyright claim beyond the public domain age so lots of due diligence is required.

The composition is mostly likely open to public domain (check your local state and national laws however):
"The Girl I left Behind", also known as "The Girl I Left Behind Me", is an English folk song dating back to Elizabethan era.[1] It is said to have been played when soldiers left for war or a naval vessel set sail. According to other sources the song originated in 1758 when English Admirals Hawke and Rodney were observing the French fleet.[1] The first printed text of the song appeared in Dublin in 1791. A popular tune with several variations, "The Girl I Left Behind Me", may have been imported into America around 1650 as 'Brighton Camp',[2] of which a copy dating from around 1796 resides in the Bodleian Library, Oxford.[3]
https://en.wikipedia.org/wiki/The_Girl_I_Left_Behind​

The cheapest option, if this song is perfect for your game, is to hire someone with decent quality virtual instruments and have them recreate the song using MIDI files which would allow you to avoid the additional fees/requirements for using a commercial recording of the song. Otherwise, your best bet is to hire someone who can create a sound-a-like where they capture the basic mood and vibe but make enough changes to avoid any issues altogether.

Licensing the actual music recording from the film will probably be the most expensive and troublesome option, honestly.

Hope that helps!

Edit: I just checked and it looks like it IS on the US list of public domain music. But just because it's there doesn't mean you're cleared to use it in your game, so please check and do enough research to protect both yourself and your work.

https://www.pdinfo.com/pd-song-list/pd-song-list-best-g.php

(scroll down to see “Girl I Left Behind (Me)”