In his report to the king, cabeza de vaca

When Cabeza de Vaca and the others reached the Spanish Empire outpost of Culiacan thousands of miles away on the west coast of Mexico in what is now the state of Sinaloa.

After of period of recovery in Culiacan, Cabeza de Vaca and the others traveled on to the city Guadalajara and from there to Mexico City. Many Spanish Empire officials recognized that Cabeza de Vaca's experience would make him extremely valuable on any future expeditions into the interior of North America. He knew several native languages and understood the cultures. Cabeza de Vaca himself wanted to go back and bring the native tribes into the Spanish Empire and convert them to Christianity by humane and enlightened means. But Cabeza de Vaca that if he were to carryout the spread of Christianity and Spanish civilization by humane means it could only occur if he were the leader of the expedition.

He undertook a perilous journey back to Spain to seek his appointment by the King to the leadership of another expedition. Unfortunately for Cabeza de Vaca and for the natives the King had already appointment Hernando de Soto to lead the next expedition. De Soto asked Cabeza de Vaca to join his expedition but Cabeza de Vaca refused. De Soto was a soldier, an accomplished military leader, and was not likely to give much credence to Cabeza de Vaca's concern for humaneness and fairness.

The King offered Cabeza de Vaca the leadership of an expedition to explore the northeastern part of North America but Cabeza de Vaca turned it down. However the King did appoint Cabeza de Vaca to leadership of an expedition but not in North America. The Spanish colony in the region of the Rio de la Plata in South America was in trouble. The Governor of the colony was missing and feared dead. Cabeza de Vaca was to go to Rio de la Plata area and seek out the missing governor and if that governor was dead Cabeza de Vaca was to take his place as governor.

op: object of preposition