[citation needed] On 14 February 1901 in Madrid, Carlos married Mercedes, Princess of Asturias, elder daughter of the late King Alfonso XII of Spain and of his wife Archduchess Maria Christina of Austria.

[citation needed] Mercedes was the elder sister and heir presumptive to King Alfonso XIII of Spain, an unmarried teenager.

Carlos served in the Spanish Army in the Spanish–American War and received the Military Order of Maria Cristina.

On marrying his first wife, Carlos renounced on 14 December 1900 his future rights of succession to the non-existent Crown of Two Sicilies in an official document, known as the Act of Cannes, subject to a requirement in the Treaty of Naples of 1759 and the Pragmatic Decree of 6 October 1759 that the Crown of Spain should not be combined with the "Italian Sovereignty".

Alfonso's claim was recognised by the heads of the different lines of the House of Bourbon, although not by the head of the Orleans family,[5] and in 1983 the Spanish Council of State, following an investigation by the Ministries of Justice and Foreign Affairs, the Royal Academy of Jurisprudence and Legislation and the Institute Salazar y Castro concluded unanimously in favour of Infante Don Alfonso's only son, Prince and Infante Don Carlos, a position shared by the Spanish Royal House.