That was an awesome (and didactic) explanation, Ace.
Let me add something to this: say that you indeed try to move to a red area, what would happen? Well, unless you manage to decrease the enemy's control points (or increase your own evasion ones), you won't be able to move forward. You will find your unit in the same region the next turn. UNLESS something has changed: the enemy has retreated, he has given battle and lost, your MC increased in the regions in question, etc.
Fortresses and fortified units are great for this role, they provide choke points that can hold the advancement of an enemy's major concentrations.
PS. In every test game I have played with Schlieffen, Liege falls next turn after the one in the screenshot, regardless what I do. The Belgian troops surrender, there is no need to assault in this very case. They are convinced by the heavy shelling.
