First Timer Supply Question

[maltshop]

[ATTACH]38617[/ATTACH]

My first campaign as the Entente. I've driven into Belgium in 1915 and freed Brussels. As you can see, I have a significant German force trapped. It looks to me like those units are out of supply. Are they actually? Is there anything else I need to do? If they are out of supply, how long does it take for them to lose their ability to fight? I'd much rather leave them there to starve than fight them.

Thanks in advance for the help!

[Altaris]

Is Simple Supply cut on in the Options? If so, it's almost impossible to cut off their supply unless you have them in a non-city region. If you want more realistic supply situations, turn off Simple Supply in the options.