However, this left the other European powers, officially, with no room for overseas expansion. While these powers refused to acknowledge the legality of the Bull and soon became involved in exploration and colonization in spite of it, they also tended to become more involved than did Portugal or Spain in some of the by-products of colonization, such as the slave trade. When the Spaniards began to use slaves in their American colonies, the Dutch, French, and British were only too eager to provide the transportation. Before long, they too had colonies and slaves of their own. The triangular trade between Europe, Africa, and the New World, was one of the most lucrative aspects of the mercantile economy. Mercantilism sought to keep each country economically self-sufficient. Within this framework the role of the colony was to provide the mother country with raw materials which it could not produce for itself and to be a market for the consumption of many of the manufactured goods produced within the mother country. This triangular trade began in Europe with the purchase of guns, gunpowder, cheap cotton, and trinkets of all kinds. These were shipped to the coast of West Africa and unloaded at a trading station. At key points along the coast, the European nations had made treaties with the local rulers allowing them to set up trading stations and slave factories. At this point, the European traders entered into hard bargaining sessions with the representatives of the local ruler in which the manufactured goods from Europe, especially guns, were traded for African slaves. When the deal was completed, the slaves were loaded on the ship, and the captain set sail for the New World. Upon arrival in the West Indies, another bargaining process was begun. Here the slaves were traded for local agricultural products which were wanted in Europe. Then the ships were loaded with tobacco, sugar, and other West Indian produce and returned to Europe for still another sale and another profit. At every point along the route, large sums of money were made. A profit of at east one hundred percent was expected. Vast wealth was obtained through the slave trade, and this money was reinvested in the developing industrial revolution. Thereby the Africans unwittingly helped to finance the European industrial revolution which widened the technological gap between Africa and Europe. The African slave was sometimes a criminal, but, more often than not, he was captured in battle. As the slave trade grew and with it the need for more slaves, the number of these battles increased. Clearly, many battles were being fought solely for the purpose of acquiring slaves who could then be sold to the European traders. Sometimes, too, the slave might have been the political enemy of the ruler or of some other powerful person.