What you describe here is pretty much the pattern of ALL wars. Resistence will slow advances while it can be maintained, but once the enemy suffers attrition to the point that equipment and/or supply can no longer resist the constant, steady pressure maintained by the attacker, they crack suddenly and the entire defense crumbles.
We arrived some days ago at the place where Iraq presents its resistence to the advance (essentially: the RG front lines), about 50 miles from Baghdad. Steady and constant pressure will be applied to that resistence through tactical maneuver and engagement, and of course air power. That resistence will hold at that point until it suffers enough attrition, or depravation of supply, that it suddently and quicky, without warning, collapses. If there is not a second line of defense, then the collapse will be total.
Nam Vet