Second Schleswig War: The Danish navy defeats the Austrian and Prussian fleets in the Battle of Heligoland.

The Second Schleswig War, also sometimes known as the Dano-Prussian War or Prusso-Danish War, was the second military conflict over the Schleswig–Holstein question of the nineteenth century. The war began on 1 February 1864, when Prussian and Austrian forces crossed the border into the Danish fief Schleswig. Denmark fought troops of the Kingdom of Prussia and the Austrian Empire representing the German Confederation.

The Austrian Empire, officially known as the Empire of Austria, was a multinational European great power from 1804 to 1867, created by proclamation out of the realms of the Habsburgs. During its existence, it was the third most populous monarchy in Europe after the Russian Empire and the United Kingdom, while geographically, it was the third-largest empire in Europe after the Russian Empire and the First French Empire.

The Kingdom of Prussia was a German state that existed from 1701 to 1918. It played a significant role in the unification of Germany in 1871 and was a major constituent of the German Empire until its dissolution in 1918. Although it took its name from the region called Prussia, it was based in the Margraviate of Brandenburg. Its capital was Berlin.

The Battle of Heligoland was fought on 9 May 1864, during the Second Schleswig War, between a Danish squadron led by Commodore Edouard Suenson and a joint Austro-Prussian squadron commanded by the Austrian Commodore Wilhelm von Tegetthoff. The action came about as a result of the Danish blockade of German ports in the North Sea; the Austrians had sent two steam frigates, SMS Schwarzenberg and Radetzky, to reinforce the small Prussian Navy to help break the blockade. After arriving in the North Sea, Tegetthoff joined a Prussian aviso and a pair of gunboats. To oppose him, Suenson had available the steam frigates Niels Juel and Jylland and the corvette Hejmdal.