Before the treachery of the Duc de Ragusa, the last hope Napoleon had in 1814 was to keep his son (Napoleon II) as Emperor of France under the Regency of his wife, the very capable Empress Marie Louise von Habsburg. However, Napoleon waited too long and the peace initiative landed in favor of the Coalition. When Napoleon returned from Elba, he wanted only one thing, for the French Empire to be reinstated under him with the borders reduced to that of 1794 (which was the original reduction of French boarders before Napoleon's return).

So, France was invaded and Napoleon rode out to fight the Prussians and British. Providing that Napoleon won, he would push for terms that would keep him on the throne of France while reuniting him with his family (his wife and son). As much of a commander, Napoleon was a man of peace that used war to promote his peace terms. The defeat of Wellington and Blucher was meant to ensure that he could live out his life in peace as ruler of France. However, no more could be said as that goes well into the realm of speculation.

Well for the battle of Waterloo directly his plan was to defeat Wellington, which in his mind meant the Prussians and British/allied armies were retreating down their respective lines of communication: the British/Dutch/Hanoverian/Belgians towards the sea, while the Prussians/Germans would retreat towards the Rhine. What he didn't realize is that the Prussians, after being defeated just prior to Waterloo at Ligny, were not fleeing with their tails between their legs, but were moving west, endangering their lines of communication, to link up with Wellington regardless.

As for Napoleon's campaign in Belgium in 1815, the Emperor was on the offensive for a few simple reasons. After Napoleon's return from Elba, he sent out diplomatic agents to the four great powers: Great Britain, Prussia, Russia and Austria to assure them he sought only peace, and that he was content with France's 1792 borders. However the allies, largely influenced by the fervently anti-Napoleon Talleyrand and Metternich, vowed to put 150,000 men each into the field to once again remove Napoleon from the throne, disregarding the fact that the great majority of the French people didn't want the Bourbons again, and in fact were more than happy to fight and die to have their own government, one with Napoleon as their Emperor.

Therefore, to face off against 600,000 men, Napoleon had a standing army of around 200,000, with additional National Guardsmen to maintain frontier fortress and city garrisons. Allied numbers would be swelling to 800,000-1,000,000 within a few months. Therefore Napoleon's only chance was to strike first, and to strike hard, defeating the Allied armies in detail before they had a chance to unite and progress onto French soil, something that had happened in 1814.

The two closest, and most powerful armies, were those of Wellington and Blucher, both stationed in Belgium. They were spread out over quite a distance, especially Wellington's troops (Blucher's were more concentrated) and between them ran the road from Charleroi to Brussels. Therefore Napoleon advanced along this axis, held Wellington off (preventing them from linking up and bringing double Napoleon's numbers down on him) at Quatre-Bras while he gave the Prussians a thrashing at Ligny. As far as Napoleon was concerned, the campaign was only to be wrapped up, he dispatched 30,000 men under Grouchy to continue pursuing the defeated Prussians, expecting them to retreat by Liege. However they instead retreated by way of Wavre and Grouchy was too late to delay them, whereupon they linked up with Wellington's left flank and defeated Napoleon who had already been lengthily engaged with the British/Hanoverian/Dutch/Belgian army.

Napoleon's plan was bold, but sound. If he were able to knock out the two most powerful and westward armies in the initial stages, as he very nearly did, then it is highly likely that he would be able to negotiate peace with Austria, who had been on its last leg with manpower since 1809 when they had spent years rebuilding their forces, only to once again be defeated by Napoleon, making it the fourth war they were to lose against France in the time of Napoleon. In addition, Napoleon was married to the Emperor Francis' daughter Marie Louise, with whom he had a son, Napoleon II, the King of Rome.

Austria's only remaining co-belligerent at this time would be Russia, a rival, who had limited tangible interests in an offensive war into France proper. Therefore the chances of a peace treaty after Blucher and Wellington's defeat were high. This was Napoleon's reasoning for conducting the campaign as he did.

Sources: History of the Waterloo Campaign , Siborne

A Military History and Atlas of the Napoleonic Wars, West Point, Elting & Esposito

1815, Waterloo: an Account of Napoleon's Last Campaign, Houssaye