A Congress had been convened at Vienna to sort out all of the difficult questions over land rights and borders after such a long war, which had seen huge changes to the political map of Europe. They were still in session at Vienna when news arrived that Napoleon had landed in France and they quickly agreed that all of the major powers would supply 150,000 men each and the smaller powers smaller contingents, in order to invade France with over three quarters of a million men to oust Napoleon once again. For the only time in history, that I am aware of, Europe declared war on one man, Napoleon!
But as can be seen, Russia and Austria would take a long time to bring their armies to the French border. Napoleon could either build huge defences around Paris and wait to be attacked, or he could try to destroy part of the huge allied army before they all managed to come up. The joint British/Hanoverian/Netherlands army (Belgium and Holland having only recently been joined as the ‘Kingdom of the Netherlands’) commanded by the Duke of Wellington and the Prussian army under Field Marshal Blucher were already stationed in Belgium, near the French border. Together their two armies heavily outnumbered his, but separately they were each inferior. Before the allies invaded, these two armies were dispersed, to enable such large numbers of troops to be fed and housed. Napoleon, saw his opportunity, if he attacked, he could destroy each of these two armies in turn and hopefully knock Britain and Prussia out of the war.