Napoleon engaged in his disastrous Russian campaign in 1812; by 1813, the military landscape was dramatically different. Prussia was the first to defect from Napoleon's cause; possibly the arrival of the remnants of the Grande Armée in Berlin convinced the Prussian leadership that it was not only safe, but expedient. The remains of Napoleon's invasion force staggered back from Russia in early 1813 in appalling condition. Of the I Corps, only 2,281 men of the original 70,000 could be mustered. The Imperial Guard strength fell from 50,000 to 1,533, and 200 of those were disabled by amputation, frostbite, or injuries. Some historians suggest that the nature of the opposition to Napoleon changed in these few months, from opposition by dynasties to opposition by whole peoples, giving the War of the Sixth Coalition its nationalist character.