15th-century Portuguese exploration of the African coast is commonly regarded as the harbinger of European colonialism, and marked the beginning of the Atlantic slave trade, Christian missionary evangelization, and the first globalization processes, which were to become a major element of European colonialism until the end of the 18th century. By the beginning of the colonial era there were forty forts operating along the coast. They acted mainly as trading posts and rarely saw military action, but the fortifications were important, as arms and ammunition were being stored prior to trade.