In 1807, Napoleonic forces seized Swedish Pomerania and forced Prussia and Russia to sign the Treaty of Tilsit. Russia was therein obliged to attack Napoleon's enemies, and since Gustav IV Adolf refused to break his alliance with the United Kingdom, the tsar invaded Finland and severed it from Sweden in the Finnish War, 1808/1809. Sweden could no longer uphold her anti-French foreign policy, and French Marshal Jean Baptiste Bernadotte was elected heir to the Swedish throne in 1810. Denmark-Norway entered an alliance with France after the second British bombardment of Copenhagen in 1807.