Rennes is the administrative capital of the French department Ille-et-Vilaine. Before the French revolution, prior to the integration of the Duchy of Brittany into the Kingdom of France, Rennes was the capital of the duchy, with the other historical capitals of Brittany's Ducal period being Nantes and Vannes. It has a long history due to its location at the confluence of two rivers and its proximity to the bordering regions from which arose various challenges to the borders of Brittany.
The eastern Armorican people of Redones founded Condate —an ancient Celtic word meaning confluent— at the confluence of the Ille and Vilaine rivers and made it the capital of a territory that extended to the Bay of Mont Saint-Michel. The name of the city of Redon also reflects that of the Redones. Early in the 1st century BCE, they adopted the Greek and Roman practice of issuing coinage, adapting the widely imitated gold staters of Philip II of Macedon, in the characteristic Celtic coin metal alloy called billion. Without inscriptions, as the Celtic practice was, the Redones coinage features a charioteer whose pony has a human head. Large hoards of their coins were unearthed in the "treasure of Amanlis" found in June 1835 and that of Saint-Jacques-de-la-Lande, discovered in February 1941. The museum at Rennes contains a large representative collection.
They joined the Gaulish coalition against Rome in 57 BC, which was suppressed by Crassus. The following year, Roman emissaries were held hostage by the Redones, which obliged Julius Caesar to intervene in Armorica and suppress the rebels, and the following year to cross the Channel to discourage further support of the Redones by the Britons. In 52 the Redones responded to the call of Vercingetorix to furnish a large contingent of warriors.
In the Roman era, Condate became Condate Riedonum, capital of Civitas Riedonum.
The oldest known Rennais is Titus Flavius Postuminus, known to us from his steles found in Rennes in 1969. As indicated by his name, he would have been born under the Flavian dynasty, under the reign of Titus, i.e. between 79 and 81 AD. One of the steles tells us, in Latin, that he took charge over all the public affairs in the Civitas Riedonum. He was twice duumvir and flamen for life for Mars Mullo.
During the Roman era, the strategic position of the town contributed to its importance. To the west the principal Roman route, via Osismii, stretched from Condate to Vorgium (modern Carhaix).
In the year 275, the threat of barbarians led to the erection of a robust brick wall around Rennes. Rennes became known as the "red town".
Threatened by the danger of peasant marauders called bagaudae at the end of the Roman Empire in the 5th century, the Armorican peninsula, including Brittany and therefore Rennes, constituted the last of the strongholds of the western Roman Empire. The invincible Armorican Romans held their ground against Clovis I, who occupied most of Alamans, then the Visigoths. Melaine, the bishop of Rennes, played an important role in the peace treaty between the Franks and the Armoricans in the year 497. He famously declared "" ("Peace must be made between Christians").
Starting in the 5th century, Bretons occupied the western part of the Armorican peninsula, which started to be called Little Britain, and then Brittany, while the Franks took the rest of Armorica. To contain the expansion and avoid Breton incursions, the Carolingians instituted a Breton March or frontier province, composed of the counties of Rennes, Nantes, and Vannes. These marches were entirely absorbed by the Breton Kingdom in the 9th century, and Rennes became fully Breton in 851. Rennes would later become the capital of the duchy of Brittany.
During the Breton War of Succession, in 1356 and 1357, the city was laid siege to by Henry of Grosmont, the Duke of Lancaster, cousin of the English king, but Bertrand du Guesclin slipped into the city and took over the resistance, which would ultimately be victorious. After nearly a year, Lancaster abandoned the English siege in 1357.
In 1491, the French army of Charles VIII, led by General Louis II de la Trémoille, unsuccessfully attacked Rennes. Brittany having already capitulated elsewhere, Rennes alone still resisted. The defenders of Rennes were determined to resist to the death, but the Duchess Anne of Brittany chose instead to negotiate. The result treaty of Rennes of November 15, 1491 concluded her marriage to Charles VIII and brought Brittany into the French kingdom. Anne jealously guarded Brittany's autonomy and the treaty promised that justice would continue to be dispensed according to practices, usages and customs maintained and observed heretofore. Furthermore, he promised the continuation of the Parlement of Brittany which met in February–April 1493, September 1494 and September 1495.
During World War II, Rennes suffered heavy damage from just three German aircraft which hit an ammunition train parked alongside French and English troop trains and near a refugee train on the yard: 1,000 died. The next day, 18 June 1940, German troops entered the city. Later, Rennes endured heavy bombings by the US and Royal Air Forces in March and May 1943, and again in June 1944, causing hundreds of deaths. Rennes contained a German transit POW camp and a POW hospital which contained many of the paratroopers captured on D-Day. Patton's army freed the capital of Brittany on 4 August, as retreating German troops blew up the bridges behind them, adding further damage. About 50,000 German prisoners were kept in four camps, in a city of only about 100,000 inhabitants at the time.
From 1954 onward, the city developed extensive building plans to accommodate upwards of 220,000 inhabitants, helping it become the third fastest-growing city in France, after Toulouse and Montpellier (1999 census).