Napoleon had been exiled to the isle of Elba after his defeat in 1814 and as European regents regained their thrones & the political class prepared for a period of peace, everyone simply believed the wars were over and tried not to think too much about the fact the ex-Emperor of France was still alive. On March 1, 1815, Napoleon escaped Elba with about 1000 loyal soldiers and landed on the Cote d'Azur. Even before radio and Internet, the news spread across the continent in a few days. At the Congress of Vienna, where leaders were leisurely re-drawing the maps of Europe, they refused to believe the rumors. It must be understood, Napoleon maintained a huge following, an almost mythical status and his reputation preceded him. The cowardly Bourbons with King Louis XVIII quickly fled France for Belgium and on March 20 the Emperor triumphantly returned to Paris. He immediately sent letters to European rulers assuring them he desired peace and that he would extend no claim over formerly conquered territory. They were having none of it. Napoleon was declared an outlaw - the monarchies undecided whether that meant anyone could kill him or if he needed to be arrested and tried first - and the nations which had defeated France a year earlier, Britain, Prussia, Russia and Austria, agreed to each send an army of 150,000 men to invade France and finish the Emperor (and the French Revolution) once and for all. Napoleon had no choice but to mobilize and re-arm.

If they other countries hadn't declared war and declared war on him, what would he have done?