War of the Castilian Succession
The War of the Castilian Succession (1475–79) provided the Catholic Monarchs with the opportunity not only to attack the main source of the Portuguese power, but also to take possession of this lucrative commerce. The Crown officially organized this trade with Guinea: every caravel had to secure a government license and to pay a tax on one-fifth of their profits (a receiver of the customs of Guinea was established in Seville in 1475—the ancestor of the future and famous Casa de Contratación).