XbattleAI did a TCP connection version, I can't remember if it was good enough. See. https://github.com/robertjschulz/xbattleai
The closed source javascript game https://generals.io is a good simplification of XBattle, I've seen one OSS server reimplementation of it.
If you can find the code for Europa I'm interested, I think it was released as OSS but I couldn't find it when looking around.
It wasn't opensource ... till the author decided to just shelve it. See above for download link.
I found the download on the wayback machine (very cool). It's basically a modernized xbattle that has a server/client that doesn't rely on X11 trusting remove server.
To download: https://web.archive.org/web/20050925030417/http://www.cgl.uw...
There was a similar game, written in java (server and client I believe), called Europa. Had a grid of X's, simple UI (tell units where to go) and the "water pressure" would slow from the cities that make armies to wherever you pointed them. Even included things like using 3 armies to make 1 paratrooper that could jump a square or two.
The author even sent me a copy, not sure I still have it though.