In March 1382, an armada under the authority of John I of Castile entered the Tagus estuary, but was unable to crack the defenses; the force advanced inland attacking, sacking and stealing cattle. King Ferdinand made António, Prior of Crato, the frontier commander. Later, the death of King Sebastian at the Battle of Alcácer-Quibir resulted in a crisis in dynastic succession between António, Prior of Crato, Catherine, Duchess of Braganza and Phillip II of Spain. From a military perspective, in order to limit support for António's claim to the crown, Phillip II determined to invade Portugal and cement his own claim to the throne. Forces from Badajoz and a Spanish fleet from Cadiz crossed the Caia ravine on 28 June 1580, where they began their invasion. António, Prior of Crato, concentrated his defense in Alcântara along the Caia ravine, where a double line of defense supported 36 carracks and nine galleons. The Portuguese troops had a static defense and could not resist a flanking move, which resulted in the loss of the battle and of Portuguese independence for the next 60 years.