Historically, the Sahara was a large barrier to aspiring conquerors - Egyptians, Romans, Carthaginians, and Arabs. Islam spread steadily... in part from the activities of Muslim traders and scholars. Explorers from Britain and France began to penetrate the Sahara in the early part of the nineteenth century. French conquests began in 1830. Political boundaries in the Sahara were defined only in the late nineteenth and the twentieth centuries. Much was left imprecise by the French, who ruled over most of the region, resulting in a number of border disputes after decolonization, including those between Morocco and Algeria over the Tindouf area, and Libya and Chad with regard to the Aozou strip.