Hypothetical Example
Imagine two equally matched sides each with two infantry and two armoured divisions. Now visualize a straight defensive line with the two infantry and two armoured divisions, deployed equally along the length of the line. Hypothetically the attacker can win by concentrating his armour at one point (with his infantry holding the rest of the line).
Traditionally it is accepted that a defending force has a 3:1 advantage over an attacker. In other words, a defending force can hold off three times its own number of attackers. Imagine then, that the defensive line is four units in length, so that each portion of the line can be held by a single defending division. Assume that they can take on the oncoming armour on equal terms (with ATGW's, pre-prepared artillery fireplans etc.) and that they have had time to dig in. This single unit should be able to hold off 3 times its own number. With the attacking force having only 2 armoured units the defenders should have the advantage.
However, as the defensive line increases from the imaginary four units in length, the advantage slips from the defender to the attacker. The longer the line to be held the thinner the defenders will be spread. With the defender having sacrificed his mobility to dig in, the attacker can choose where and when to attack. Either penetrating the line or turning a flank and thus being able to destroy the enemy in detail. Thus, concentrating two divisions and attacking at a single point generates a far greater force than is achieved by spreading two divisions into a line and pushing forward on a broad front.
Concentration of force in this scenario requires mobility (to permit rapid concentration) and power (to be effective in combat once concentrated). The tank embodies these two properties and for the past seventy years has been seen as the primary weapon of conventional warfare.
No one side has a monopoly on military art, and what is obvious to one side is obvious to the other. A far more likely scenario is that both forces will choose to use their infantry to hold a line and to concentrate their armour, however rather than a line in the sand, the infantry line would be more of a trip wire, to warn of where the enemy has chosen to launch his attack, with the armoured forces jostling to find the right place to attack or counterattack. Other considerations then must come into play for a decisive blow to be achieved.
Such considerations may be economic or political in nature. e.g. one side is unable or unwilling to allow the sanctity of its soil to be violated, and thus insists on defending a line on a map; most nations who have fallen victim to this vice have lost wars.
Read more about this topic: Force Concentration