In order that the campaign may be understood, of which Waterloo was the climax, it must be always remembered that political reasons rendered it essential for Napoleon to assume the offensive in spite of inferior numbers, and offered every inducement to the allies to await attack, and also that this gave Napoleon the great strategical advantage of the initiative. The allies had to guard the Belgian frontier; he could select his own point for invasion. Accordingly the allies occupied a line from east to west, some thirty miles south of Brussels, and a little north of the actual frontier. The Prussians on the eastern half lay chiefly on the north side of the Meuse and its tributary the Sambre.
Wellington's army was only a third English, another third being Dutch-Belgians of very poor quality and doubtful fidelity,[72] the remainder Germans, some of them excellent troops, the rest mere recruits. He covered the western part of the frontier, as was natural, seeing that he drew his supplies from England by the Belgian ports of Ostend and Antwerp. The two armies met just south of Brussels, near Charleroi, where a main road crosses the Sambre. The allies could not of course know by what route Napoleon, if he assumed the offensive, as was probable, would enter Belgium. They therefore had to watch the whole line; and partly for this reason, partly for convenience of subsistence, they quartered their forces over a space of country fully 100 miles in length from east to west. On the Prussian side, where the rivers formed a protection, and where there was less reason to expect attack, the troops were comparatively near the frontier line. On the English side none except outposts were close to the frontier, and some were at least thirty miles behind it.
Napoleon could bring into the field about 125,000 men, practically all veterans commanded by excellent officers, though the successive re-organisations after his first overthrow and on his restoration had left it lacking in the perfect mutual confidence of officers and men which makes a veteran army so formidable. Of these nearly 24,000 were cavalry; and there were 344 field guns. The Prussians under Blucher were not much inferior in numbers to the French, about 121,000, but their proportion of cavalry (12,000) and guns (312) was lower, and the quality of part at least of the army inferior. Wellington had about 94,000 men, of whom over 14,000 were cavalry, with 196 guns, but, as has been said before, barely half[73] of them were really trustworthy troops. Thus Napoleon was nearly equal to his opponents in number of cavalry, but was outmatched in guns in the proportion of about three to two, and in infantry by at least seven to four, though the superior quality of his troops went some way towards compensating for this inequality. He had the further advantage of unity of command, while the armies of the allies not only were separate, with no further concert of action than what the voluntary accord of the chiefs might establish, but drew their supplies from opposite directions, the Prussians from the Rhine, the English from the Belgian ports. Both sides seem to have been fairly well informed as to the strength of the other. Napoleon also had information as to the position of the allied troops, nor were Wellington and Blucher quite in the dark when the French troops concentrated on the frontier, skilfully as Napoleon had arranged their movements, though they could not at first be certain what was real attack, what feint.