Sweden declares war on its ally the United Kingdom to begin the Anglo-Swedish War, although no fighting ever takes place.

Anglo-Swedish war of 1810–1812

Theoretical state of war between Sweden and UK in Napoleonic times

During the Napoleonic Wars until 1810, Sweden and the United Kingdom of Great Britain and Ireland were allies in the war against Napoleon. As a result of Sweden's defeat in the Finnish War and the Pomeranian War, and the following Treaty of Fredrikshamn and Treaty of Paris,…