Here, then, we have Napoleon on the morning of Friday the 16th of June prepared to deal with the Prussians. It is his right-hand body, under Grouchy, which is deputed to do this, while he sends up the left-hand body, under Ney, northwards to brush aside, or, at the worst, at least to hold off whatever of the Duke of Wellington’s command may be found upon the Brussels road attempting to join the Prussians.
The general plan of what happened upon that decisive 16th is simple enough.
The left-hand body, under Ney, goes forward up the Brussels road, finds more resistance than it expected, but on the whole performs its task and prevents any effective help being given by the western half of the Allies—Wellington’s half—to the eastern half—the Prussian half. But it only prevents that task with difficulty and at the expense of a tactical defeat. This action is called Quatre Bras.
Meanwhile, the right-hand body equally accomplishes the elements of its task, engages the head of the Prussian line and defeats it, with extreme difficulty, just before dark. This action is called Ligny.
But the minor business conducted by the left, under Ney, is only just successful, and successful only in the sense that it does, at vast expense, prevent a junction of Wellington with Blucher. The major business conducted on the right, by Napoleon himself, in support of Grouchy, is disappointing. The head of the Prussian line is not destroyed; the Prussian army, though beaten, is free to retreat in fair order, and almost in what direction it chooses.
The ultimate result is that Wellington and Blucher do manage to effect their junction on the day after the morrow of Ligny and Quatre Bras, and thus defeat Napoleon at Waterloo.
Now, why were both these operations, Quatre Bras and Ligny, incompletely successful? Partly because there was more resistance along the Brussels road than Napoleon had expected, and a far larger body of Prussians in front of him than he had expected either; but much more because a whole French army corps, which, had it been in action, could have added a third to the force of either the right or the left wing, was out of action all day; and wandered aimlessly over the empty zone which separated Ney from Grouchy, Quatre Bras from Ligny, the left half of Napoleon’s divided army from its right half.
This it was which prevented what might have been possible—the thrusting back of Wellington along the Brussels road, and even perhaps the disorganisation of his forces. This it was which missed what was otherwise certainly possible—the total ruin of the Prussian army.
This army corps thus thrown away unused in hours of aimless marching and countermarching was the First Army Corps. Its commander was Erlon; and the enormous blunder or fatality which permitted Erlon and his 20,000 to be as useless upon the 16th of June as though they had been wiped out in some defeat is what makes of the 16th of June the decisive day of the campaign.