Napoleon, on his return from Elba in March 1815, finding that his overtures to the European Powers were without avail, and not daring to risk another defensive war, decided on assuming the offensive. For many reasons, the allied armies in Belgium offered him the most advantageous point to aim at, since, from their position and propinquity, they menaced Paris; also the Belgian nation was sympathetic to his cause, and if he could defeat the British and Prussians and possess himself of the Rhine frontier, he would be on the flank of the advancing hosts of Russia and Austria, and in the most favourable position for making peace on advantageous terms.

Having concentrated over 92,000 infantry, 22,000 cavalry, and 350 guns on the frontier, he suddenly broke in on 15th June, and the following day defeated the Prussians at Ligny with his main body, whilst his left wing, under Ney, fought an indecisive action with the British at Quatre Bras. After despatching Grouchy to pursue the Prussians, he turned with all his force on Wellington, who fell back to the position of Waterloo.

Here, on 18th June, the decisive battle was fought, the British and Hanoverians sustaining the repeated attacks of Napoleon throughout the day. The arrival of the Prussians on the right flank of the French during the afternoon greatly relieved the pressure on the British position, and towards evening the French, having been repulsed in all their attacks on Wellington, and being taken in flank by Blucher, were completely defeated and driven off the field of battle.

The Allies marched on Paris, which they entered on 7th July.