Following the collapse of the Roman Empire, Cantabria regained its independence from the rule of the Visigoths. In 574, King Liuvigild attacked Cantabria and managed to capture the south of the country, including the city of Amaya, where he established a Visigothic province called the Duchy of Cantabria (see picture), which would serve as a limes or frontier zone to contain the Cantabri as well as their neighbors the Vascones. To the north of this cordon, however, the Cantabri continued to live independently until the Arab invasion. In 714, a mixed Arab/Berber army of Muslim Moors invaded the upper valleys of the Ebro and succeeded in capturing Amaya, the Cantabrian capital, forcing the Cantabrians back to their traditional frontiers, where they joined forces with the Kingdom of Asturias. In the first chronicles of the Reconquista, Cantabria still appears to be acknowledged as a region. In the Albendense Chronicle, when speaking of Alfonso I, it says, “This was the son of Peter, the duke of Cantabria”.

During the 9th century, on mentioning the monastery of Saint Zacharias, Eulogius pinpoints it in Seburim (maybe Zubiri) on the river Arga, “waters all of Cantabria”, in a letter sent to the bishop of Pamplona Williesind, suggesting a region stretching out far into the east. From this period on, source documents barely reference Cantabria by name, with Asturias featuring in names of the comarcas called Asturias de Santillana, Asturias de Trasmiera and Asturias de Laredo.

From a central core formed by the Hermandad de las Cuatro Villas (Brotherhood of the Four Cities) (Santander, Laredo, Castro Urdiales and San Vicente de la Barquera), the Hermandad de las Marismas (Brotherhood of the Marshes) was created, thereby uniting all the important seaports to the East of Asturias. During the period of the Reconquista, the Four Cities actively participated in the re-settling of Andalusia, dispatching men and ships. The coastal port cities of Cádiz and El Puerto de Santa María were settled by families from the Cantabrian Sea ports. Ships from the Four Cities took part in the taking of Seville, destroying the ship bridge linking Triana and Sevilla, a victory that is represented by the Carrack and the Torre del Oro of Sevilla in the coat of arms of Santander, Coat of arms of Cantabria and Avilés (Asturias).