An equilibrium can seldom be sustained for more than six or seven hours between forces on the field of battle; but in this instance, the state of the ground rendered the movements so slow as to prolong the battle for about twelve hours; thus enabling the allies to effect a concentration in time to save Wellington.
Many of Napoleon's brilliant victories resulted from merely bringing troops to bear suddenly upon some decisive point.

Rivoli in 1796-7, Marengo in 1800, Ulm in 1805, Jena in 1806, Ratisbon in 1809, Brienne in 1814, and Ligny in 1815, are familiar examples.

But this concentration of forces, even with a regular army, cannot be calculated on by the general with any degree of certainty, unless his communications are perfectly secure.

And this difficulty is very much increased where the troops are new and undisciplined.

When a country like ours is invaded, large numbers of such troops must suddenly be called into the field.