He tried to divert public opinion from Finland to Norway, by arguing that to create a compact peninsula, with sea for its natural boundary, was to inaugurate an era of peace, and that waging war with Russia would lead to ruinous consequences. Soon after Charles John's arrival in Sweden, Napoleon compelled him to accede to the Continental System and declare war against Great Britain; otherwise, Sweden would have to face the determination of France, Denmark and Russia. This demand would mean a hard blow to the national economy and the Swedish population. Sweden reluctantly declared war against Great Britain but it was treated by both countries as being merely nominal, although Swedish imports of British goods decreased from £4,871 million in 1810 to £523 million in the following year. In January 1812, French troops suddenly invaded Swedish Pomerania and the island of Rügen. The decisive reason was that Napoleon, before marching to Moscow, had to secure his rear and dared not trust a Swedish continental foothold behind him. To render it the more insulting, Napoleon scheduled it for the Crown Prince's birthday. The initially amicable relationship which Charles John had with Napoleon soon changed because of this invasion. The invasion was a clear violation of international law as well as an act of war so public opinion in Sweden was understandably outraged. Moreover, it antagonized the pro-French faction at the Swedish court. Thereafter, the Crown Prince declared the neutrality of Sweden and opened negotiations with Great Britain and Russia. In 1813, he allied Sweden with Napoleon's enemies, including Great Britain, Russia and Prussia, in the Sixth Coalition, hoping to secure Norway. After the defeats at Lützen (2 May 1813) and Bautzen (21 May 1813), it was the Swedish Crown Prince who put fresh fighting spirit into the Allies; and at the conference of Trachenberg he drew up the general plan for the campaign which began after the expiration of the Truce of Pläswitz. Charles John, as the Commander-in-Chief of the Northern Army, successfully defended the approaches to Berlin and was victorious in battle against Oudinot in August and against Ney in September at the Battles of Großbeeren and Dennewitz. Bernadotte's Army of the North would continue to guard Berlin and keep watch on Davout's forces in Hamburg while the Allies, in accordance with the plan conceived at Trachenberg, maneuvered toward Napoleon's army at Leipzig. With the other Allied armies engaged in battle on 17 October, Bernadotte's army finally crossed the Elbe and joined in the Battle of Leipzig on 19 October.