The idea that a war can be won by standing on the defensive and waiting for the enemy to attack is a dangerous fallacy, which owes its inception to the desire to evade the price of victory.
Further, a defensive policy involves the loss of the initiative, with all the consequent disadvantages to the defender.
So long as the opposing forces are at the outset approximately equal in numbers and moral and there are no flanks to turn, a long struggle for supremacy is inevitable.
Obviously, the greater the length of a war the higher is likely to be the number of casualties in it on either side.
Once the mass of the defending infantry become possessed of low moral, the battle is as good as lost.