Units leaving "the main front line"?

[Syt]

Playing as Entente, 1914. I have mandatory attacks into Alsace Lorraine that I executed, having one army in Metz, the other in Thionville.

During the events phase I am told that 1st and 2nd Army have left the main front lines (I suffer only 0 points National Will, though). How do I know where the main front line is?

Here's the situation:

[calvinus]

Mmmm... I remember the concerned front area is the Vosges...

[Syt]

So what should I do when I have a mandatory attack there that succeeds? Retreat back to cover that front line? Is it ok to put *any* unit there? E.g. detach one corps and have the rest move on? Just trying to understand what I am supposed to do.