I've never tried the Commandos multiplayer, but it sounds like worth a shot so I thought I'd ask if anyone was interested to try it out right this moment. I mean either Behind Enemylines or Beyond the Call of Duty.

This is relevant to anyone interested to try the multiplayer (pasted from Steam forums):

"It took me a while to figure it out and get it working.
For starters, you can find a special program in your installation folder of Commanders BEH inside your Steam folder. You can find it in Steam/steamapps/common/commandos behind enemy lines.
The program you are looking for is 'mpserver.exe'. Starting this will run a dedicated server. This game is old and has no "lobby" or automatic hosting options inside the game itself like most modern games have. Ah... the good old days...

Anyway, in the information window of the dedicated server you will be given an IP-address to connect to. If you start your game while having the dedicated server running in the background, you can join your own started server by filling in the given IP address in the multiplayer screen.
If you give the same IP-address to other people on the internet (or your network) they will be able to connect.
Note: this IP should most likely be your external IP-adress. If you don't know that, just Google 'what is my ip'.

Now, there is a catch. If your computer is behind a modem or router, the game will not be able to work with that. It does not know how to enter a network and find a game-server somewhere on there, it only knows how to connect straight to another computer. You will know you have this problem if the given IP in the dedicated server window is a LAN-ip instead of your external internet IP.
To still be able to play online without unplugging the router/modem you need to forward two ports to your personal computer on your router/modem. The ports you have to forward are 4321 and 4322 in TCP mode. You forward them to your computer's LAN-ip so that the searching game on the internet will be directed straight to your computer.

For me, this worked straight away. I have tested it with a friend of mine and it worked fine.
Now, I realise that this is probably complex to do. I kind of hope you are experienced with your system.
If you have questions, don't hesitate to ask them.
Remember that this forum is also a good place to look around. Trust me, you are not the only one with this problem and others will have found solutions too. "