HIS: This Day in History: 1870 – Emperor Napoleon III of France is deposed and the Third Republic is declared.

 HIS: This Day in History: 1870 – Emperor Napoleon III of France is deposed and the Third Republic is declared.

The French Third Republic (French: La Troisième République, sometimes written as La IIIe République) was the system of government adopted in France from 1870, when the Second French Empire collapsed during the Franco-Prussian War, until 10 July 1940 after France's defeat by Nazi Germany in World War II led to the formation of the Vichy government in France.

The early days of the Third Republic were dominated by political disruptions caused by the Franco-Prussian War of 1870–71, which the Republic continued to wage after the fall of Emperor Napoleon III in 1870. Harsh reparations exacted by the Prussians after the war resulted in the loss of the French regions of Alsace (keeping the Territoire de Belfort) and Lorraine (the northeastern part, i.e. present-day department of Moselle), social upheaval, and the establishment of the Paris Commune. The early governments of the Third Republic considered re-establishing the monarchy, but disagreement as to the nature of that monarchy and the rightful occupant of the throne could not be resolved. Consequently, the Third Republic, originally envisioned as a provisional government, instead became the permanent form of government of France.

The French Constitutional Laws of 1875 defined the composition of the Third Republic. It consisted of a Chamber of Deputies and a Senate to form the legislative branch of government and a president to serve as head of state. Calls for the re-establishment of the monarchy dominated the tenures of the first two presidents, Adolphe Thiers and Patrice de MacMahon, but growing support for the republican form of government among the French populace and a series of republican presidents in the 1880s gradually quashed prospects of a monarchical restoration.

The Third Republic established many French colonial possessions, including French Indochina, French Madagascar, French Polynesia, and large territories in West Africa during the Scramble for Africa, all of them acquired during the last two decades of the 19th century. The early years of the 20th century were dominated by the Democratic Republican Alliance, which was originally conceived as a centre-left political alliance, but over time became the main centre-right party. The period from the start of World War I to the late 1930s featured sharply polarized politics, between the Democratic Republican Alliance and the Radicals. The government fell less than a year after the outbreak of World War II, when German forces occupied much of France, and was replaced by the rival governments of Charles de Gaulle's Free France (La France libre) and Philippe Pétain's Vichy France (L'État français).

Adolphe Thiers called republicanism in the 1870s "the form of government that divides France least"; however, politics under the Third Republic were sharply polarized. On the left stood Reformist France, heir to the French Revolution. On the right stood conservative France, rooted in the peasantry, the Roman Catholic Church and the army. In spite of France's sharply divided electorate and persistent attempts to overthrow it, the Third Republic endured for seventy years, which as of 2020 makes it the longest lasting system of government in France since the collapse of the Ancien Régime in 1789.