Napoleon was at Bamberg, waiting to see what the Prussians intended, Andrew Roberts explains (in Napoleon: A Life), when the Prussian declaration of war arrived:
‘He threw it away contemptuously,’ recalled Rapp, and said of Frederick William, ‘Does he think himself in Champagne?’ — a reference to the Prussian victories of 1792. ‘Really, I pity Prussia. I feel for William. He is not aware what rhapsodies he is made to write. This is too ridiculous.’
His reply:
Your Majesty will be defeated, you will compromise your repose and the existence of your subjects without the shadow of a pretext. Prussia is today intact, and can treat with me in a manner suitable to her dignity; in a month’s time she will be in a very different position. You are still in a position to save your subjects from the ravages and misfortunes of war. It has barely started, you could stop it, and Europe would be grateful to you.
He had his reasons for being confident:
Although Prussia had a potentially very large army of 225,000 troops, 90,000 of them were tied up garrisoning fortresses. No immediate help could be expected from Russia or Britain, and although some of her commanders had fought under Frederick the Great, none had seen a battlefield in a decade. Her commander-in-chief, the Duke of Brunswick, was a septuagenarian and her other senior commander, General Joachim von Möllendorf, an octogenarian. Moreover, Brunswick and the general in charge of the left wing of the Prussian army, Prince Friedrich von Hohenlohe, had rival strategies and hated each other, so councils of war could take up to three ill-tempered days to reach a conclusion. Napoleon didn’t hold a single council of war throughout the entire campaign.