The Sonderbund War (German: Sonderbundskrieg, French: Guerre du Sonderbund, Italian: Guerra del Sonderbund) of November 1847 was a civil war in Switzerland, then still a relatively loose confederacy of cantons. It ensued after seven Catholic cantons formed the Sonderbund ("separate alliance") in 1845 to protect their interests against a centralization of power. The war concluded with the defeat of the Sonderbund. It resulted in the emergence of Switzerland as a federal state, concluding the period of political "restoration and regeneration" in Switzerland.

The Sonderbund consisted of the cantons of Lucerne, Fribourg, Valais, Uri, Schwyz, Unterwalden and Zug, all predominantly Catholic and governed by conservative administrations. The cantons of Ticino and Solothurn, also predominantly Catholic but governed by liberal administrations, did not join the alliance.

After the Tagsatzung (Federal Diet) declared the Sonderbund unconstitutional (October 1847) and ordered it dissolved by force, General Guillaume Henri Dufour led the federal army of 100,000 and defeated the Sonderbund forces under Johann Ulrich von Salis-Soglio in a campaign that lasted only a few weeks, from November 3 to November 29, and cost about 100 lives. Dufour ordered his troops to care for the injured, anticipating the formation of the Red Cross in which he participated a few years later. Major actions were fought at Fribourg, Geltwil (12 November), Lunnern, Lucerne, and finally at Gisikon (23 November), Meierskappel, and Schüpfheim, after which Lucerne capitulated on 24 November. The rest of the Sonderbund surrendered without armed resistance in the subsequent weeks.

The radical (progressive liberal) Free Democratic Party of Switzerland (German: Freisinnig-Demokratische Partei, French: Parti radical-démocratique), which was mainly made up of urban bourgeoisie and burghers and was strong in the largely Protestant cantons, obtained the majority in the Federal Diet (the Tagsatzung) in the early 1840s. It proposed a new Constitution for the Swiss Confederation which would draw the several cantons into a closer relationship. In 1843, the conservative city patricians and mountain or Ur-Swiss from the largely Catholic cantons were opposed to the new constitution. These cantons combined to form the Sonderbund in 1845. In addition to the centralization of the Swiss government, the proposed new Constitution also included protections for trade and other progressive reform measures.

The Sonderbund alliance was concluded after the Federal Diet, with the approval of a majority of cantons, had taken measures against the Roman Catholic Church such as the closure of monasteries and convents in Aargau in 1841, and the seizure of their properties. When Lucerne, in retaliation, recalled the Jesuits to head its education the same year, groups of armed radicals (Freischärler) invaded the canton. This caused a revolt, mostly because rural cantons were strongholds of ultramontanism.

The liberal majority in the Tagsatzung voted to order the Sonderbund dissolved on October 21, 1847; it deemed the Sonderbund a violation of section 6 of the Federal Treaty of 1815, which expressly forbade such separate alliances. The confederate army was raised against the members of the Sonderbund. The army was composed of soldiers of all the other cantons except Neuchâtel and Appenzell Innerrhoden (which remained neutral).

By the Treaty of Vienna of 1815, the major powers guaranteed the new Swiss Constitution and had a right to intervene if they all agreed it was necessary. At this point, Austria and France were conservative Catholic powers and wanted to help the Swiss conservatives. Austria did provide some money and munitions, but bickered with France on exactly what to do. When they finally did agree, Lord Palmerston, Prime Minister of Britain, vetoed any intervention, because he favored the liberal cause and wanted the Jesuits expelled. There was no significant foreign intervention.

The question of command remained long unsettled with the Sonderbund. The coalition's strong man, Constantin Siegwart-Müller of Lucerne, first considered appointing a foreigner (Dezydery Chłapowski of Poland or Friedrich von Schwarzenberg of Austria were mentioned), but the allied council insisted on a Swiss commander. General Ludwig von Sonnenberg and Colonel Philippe de Maillardoz of Fribourg were considered, but ultimately the council elected Guillaume de Kalbermatten of Valais. After Kalbermatten declined the appointment (he would later command the troops of Valais), Colonel Jean-Ulrich de Salis-Soglio of Grisons was elected and sworn in as commander in chief on 15 January 1847. He appointed Franz von Elgger as chief of staff. Although a Protestant himself, Salis-Soglio was a staunch Conservative and an opponent of the liberal Radicals who now controlled the "rump Confederation".