But although the allied armies thus formed together the arc of a large circle covering Brussels, they were entirely distinct.

The British drew their supplies from Ostend, on the right of their position, while Liege on the extreme left was the base of the Prussians.
Napoleon's movements were uncertain.

He might either advance upon Namur and cut off the Prussians from their base, or between Grammont and Oudenarde, by which measure he would similarly cut the British off from Ostend; or he might advance from Charleroi direct upon Brussels, breaking through at the point where Wellington's left joined the Prussian right.

The Duke of Wellington believed that he would attempt the second of these alternatives, as in that case he would fall upon the British before the Prussians could come up to their assistance, and if successful would not only cut them off from the base of supplies, but would be able to march straight upon Brussels.

It was to defeat this plan that the duke posted the largest proportion of his British troops along the frontier, holding, however, two British divisions and the Brunswick and Nassau troops in and round Brussels, where they were nearly equidistant from any point that could be attacked, and could be moved forward as soon as the enemy's intentions became manifest.
By the time that the whole of the forces were assembled Wellington had ninety thousand men under his orders; Blucher, the Prussian general, had one hundred and sixteen thousand; while Napoleon had one hundred and twenty-five thousand with which to encounter this vastly superior force.