Bit of background: I'm making a text-based game similar to warlight, and it involves territories attacking eachother and losing armies.

I'd like to know how warlight works out how many armies will be left in territory A and B after territory A attacks territory B where...

A attacks with x armies (ignoring how many other armies it has), and
B has y armies to defend with.

The result would be that

a armies retreat back to A and
B is left with b armies.

I tried looking at 'analyse attack' for help but i just got extra bamboozled.

It's the very first link on the Help tab.

There's a couple of examples on https://www.warlight.net/Forum/1120-fixing-cumulative-luck-calculation on the first post.

What language are you using BTW?

I'm using java, its not a really big project, just something to keep me occupied over the summer.
Somehow I didn't think to look on the help tab / wiki