[1] Wilhelm retired from Prussian military service in 1863 with the rank of Lieutenant General, shortly before his marriage to Princess Maria of Leuchtenberg.

[1][3] In 1866, during the Austro-Prussian War between the Kingdom of Prussia and the Austrian Empire, Wilhelm assumed command of the Baden Division of the 8th Federal Corps (German: 8.

[8] The resistance was not easily subdued and the Germans suffered heavy losses, however according to historian Gustave Louis Maurice Strauss, "[Wilhelm] carried the heights of St. Apollinari in gallant style and occupied the suburbs from which the Germans ultimately forced their way into the city where fierce fights from barricade to barricade from house to house lasted till midnight.

[1][2][9][10][11] In 1895, Kaiser Wilhelm II promoted him à la suite to the Grenadier Regiment (German: Leibgrenadierregimentes) in honor of the 25th anniversary of the Battle of Nuits-Saint-Georges.

She was the eldest surviving daughter of Maximilian de Beauharnais, 3rd Duke of Leuchtenberg and his wife Grand Duchess Maria Nikolaevna of Russia, Russian Empire.

[1][2] Upon learning of the marriage, United States President Abraham Lincoln sent a letter to Wilhelm's elder brother Frederick I, Grand Duke of Baden in which Lincoln stated: "I participate in the satisfaction afforded by this happy event and pray Your Royal Highness to accept my sincere congratulations upon the occasion together with the assurances of my highest consideration.

[15][16][17] The Russian Empire's preferred candidate for the Greek throne fluctuated between Nicholas de Beauharnais, 4th Duke of Leuchtenberg and Wilhelm, his brother-in-law.

[18] According to The New York Times on 16 March 1863, then recent purchases of Greek bonds in London were the result of a report that Wilhelm was to be formally recommended for the throne.