the corps which Erlon commanded was the _First_
Corps; for, as we shall see, upon Erlon's wanderings with this First
Corps depended the unsatisfactory termination of Ligny, and the subsequent
intervention of the Prussians at Waterloo, which decided that action.

It is also of little moment for the purpose of this to retain the names of
the places which were the headquarters of each of these corps before the
advance began. It is alone important to the reader that he should have a
clear picture of the order in which this advance took place, for thus only
will he understand both where it struck, and why, with all its rapidity,
it suffered from certain shocks or jerks.

Napoleon's advance was upon three parallel lines and in three main bodies.

The left or westernmost consisted of the First and Second Corps d'Armee;
the centre, of the Imperial Guard, together with the Third and Sixth
Corps. The third or right consisted of the Fourth Corps alone, with a
division of cavalry. These three bodies, when the night of Wednesday the
14th of June fell, lay, the first at Sorle and Leer; the second at
Beaumont, and upon the road that runs through it to Charleroi; the third
at Philippeville.

It is at this stage advisable to consider why Napoleon had chosen the
crossing of the Sambre at Charleroi and the sites immediately to the north
on the left bank of that river as the point where he would strike at the
long line of the Allies.

Many considerations converged to impose this line of advance upon
Napoleon. In the first place, it was his task to cut the line of the
Allies in two at the point where the extremity of one army, the Prussian,
touched upon the extremity of the other, that of the Duke of Wellington.
This point lay due north of the river-crossing he had chosen.

Again, the main road to Brussels was barred by the fortress of Mons,
which, though not formidable, had been put in some sort of state of
defence.

Again, as a glance at the accompanying map will show, the Prussian half of
the allied line was drawn somewhat in front of the other half; and if
Napoleon were to attack the enemy in detail, he must strike at the
Prussians first. Finally, the line Maubeuge-Philippeville, upon which he
concentrated his front, was, upon the whole, the most central position in
the long line of his frontier troops, which stretched from Metz to the
neighbourhood of the Straits of Dover. Being the most central point, not
only with regard to th