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.