17 October, Bernadotte's army finally crossed the Elbe and joined in the Battle of Leipzig on 19 October. His fresh troops, reinforced by 30,000 Prussians, joined the fray against the already battered French lines where Swedish forces entered battle in numbers for the first time in the campaign. At a critical moment entire Saxon regiments went over to his army in response to a proclamation released a week prior in which Bernadotte invited the Saxons to join their old commander in defeating Napoleon. The Army of the North committed the coup de grâce on the already depleted French and Bernadotte was the first of the Allied sovereigns to enter Leipzig. After the Battle of Leipzig he went his own way, determined at all hazards to cripple Denmark and to secure Norway, defeating the Danes in a relatively quick campaign. His efforts culminated in the favourable Treaty of Kiel, which transferred Norway to Swedish control. The Norwegians rejected Swedish control. They declared independence, adopted a liberal constitution and elected Danish crown prince Christian Frederick to the throne. The ensuing war was swiftly won by Sweden under Charles John's generalship. The military operations in 1814 were to be Sweden's last war to this day. Charles John could have named his terms to Norway, but in a key concession accepted the Norwegian constitution and its own political autonomy. This paved the way for Norway to enter a personal union with Sweden later that year. During the period of the Allied invasion of France in the winter and spring of 1814, when it was unclear who would rule France after the war, the Russian Tsar Alexander I, with support from French liberals like Benjamin Constant and Madame de Staël, advocated placing Charles John on the French throne in place of Napoleon. He would rule France as a Constitutional Monarch and leave Oscar as Regent in Sweden and Norway. Ultimately the British and Austrians vetoed the idea, and the Allies agreed that if Napoleon were to be deposed, the only acceptable alternative was the restoration of the House of Bourbon. King of Sweden and Norway Charles John had been regent and de facto head of state upon his arrival, and took an increasing role in government from 1812 onward, with Charles XIII reduced to a mute witness in government councils following a stroke. Upon Charles's death on 5 February 1818, Charles John ascended the throne.