Almost 500 years ago, the newly formed Empire of Spain — forged by the marriage of Ferdinand and Isabella, blessed by the pope, and unified by the eviction of Moors and the forcible conversion of Jews — issued a declaration that would forever define the Americas. It was called the Requerimiento, and it stated unequivocally to natives of the New World that the land on which they stood belonged to the Spanish Crown, that they would henceforward be Christians, and that any effort to resist would be met with war, seizure and enslavement. Never mind that no indigenous American understood what was being said. Never mind that the declaration went unheard as it was shouted from hilltops, bellowed from ships or roared as conquerors gave chase after fleeing Indians. It was law in the Americas, and though called into question by conscientious Spaniards and human rights activists of the day, it became the legal basis for rampant bloodshed and slavery.

Few made efforts to spare the cruelty. One of the most notable defenders of Indians in Americas North and South was the explorer Álvar Núñez Cabeza de Vaca, a Spaniard who earned his sympathies the hard way: He had once been a slave to Indians himself.

The story of Cabeza de Vaca makes for almost hallucinatory reading: a dreamlike journey that ranges from the marshes of North America to the pampas of South America. In it, one man battles New World and Old in a desperate attempt to bring the two together. Told admirably in a new biography by Robin Varnum, it is a startling chronicle of “first contact” — when cultures face one another for the first time.

Nothing had predisposed Cabeza de Vaca to commiserate with “the conquered.” His maternal grandfather, the hidalgo Pedro de Vera, had made a name for himself by plundering African cities and bringing booty and slaves to Castile. Eventually, Vera was contracted by the queen to conquer the Canary Islands, and conquer them he did: He burned aboriginals at the stake and hustled captives into slavery. When islanders rose up in revolt, he slaughtered all males over the age of 15 and sold women and children into bondage.

Little in Cabeza de Vaca’s childhood indicated that there was something wrong in this. He was proud of his grandfather. And, like any lad of his place and time, he was exhilarated by the epic adventures of Christopher Columbus. There was an empire to make, infidels to repulse, a faith to spread. And so, before he was a full adult, Cabeza de Vaca entered the service of his country. It was 1511, Queen Isabella was dead, and King Ferdinand was preparing to issue the Requerimiento.

’Álvar Núñez Cabeza de Vaca: American Trailblazer’ by Robin Varnum (Univ. of Oklahoma)

Cabeza de Vaca fought for the Spanish Crown in Italy. He helped put down the rebellious Comuneros in Seville. But news about Cortés’s breathtaking conquest of the Aztecs had captured his generation’s imagination. By 1522, Magellan’s expedition had successfully circumnavigated the globe, and Spanish colonies were sprouting across the Americas — Hispaniola, Cuba, Puerto Rico, Jamaica, Panama, Mexico. A young man’s opportunities seemed boundless. Gold was the immediate lure, but there were other treasures to be had: silver, pearls, emeralds, tobacco, cacao, sugar. Or simply a comfortable hacienda in a land of plenty.

In early 1527, Cabeza de Vaca joined an expedition under the leadership of veteran explorer Pánfilo de Narváez. Capt. Narváez was a plantation owner in Cuba, a trusted servant of the Crown. He had been ordered to chase after Cortés when the renegade conquistador countermanded orders, scuttled his own ships and forced his men to march into the heart of Mexico. Narváez’s mission now was to explore, conquer and settle the virgin land known as La Florida. Cabeza de Vaca was appointed as royal treasurer to his fleet.

The story of the Nárvaez expedition reads like Homeric myth complete with omens. As Varnum tells it: “One of the ten married women sailing with the expedition took the precaution of consulting a fortune teller. The seer, a Moorish woman from Hornachos, warned her client that all or almost all of those who followed Narváez into the North American interior would die. The seer hinted, however, that God might spare one man and work great miracles through him. . . . Apparently none of the men took it seriously.”

They should have. Indeed, after a debilitating series of storms and losses at sea, Cabeza de Vaca and his shipmates finally came ashore north of Tampa Bay. Bedraggled, starving, reduced to a fraction of their original number, they quickly learned as they lumbered their way through swamps in heavy armor that food would be scarce and natives unwelcoming. By then, Spanish law had been modified to call for a gentler Conquista. Spaniards were to declare war and enslave the indigenous only if it was certain that they worshipped idols, rejected Christ or ate human flesh. As the Narváez expedition splintered — some hacking their way inland, others limping up the coast — the Indians they encountered alternately fed, fought , led and misled them.

Desperation became such that Narváez lost all ability to command. He told his crew that it was every man for himself now. Months went by, and they were still lost, still hungry, still insanely looking for gold. Worst of all, a brutal winter was settling in. The castaways began to perish of starvation and disease. Eventually, they broke their own law and began eating human flesh. Cabeza de Vaca was soon on his own, staggering through the wilderness, until he was taken in as a slave, first by the Indians of Malhado and then by a series of benevolent tribes as he made his way on an eight-year journey that took him 2,500 miles — rounding the Gulf Coast to Texas, across northern Mexico and down the Sonoran Desert to Mexico City.

Along the way, he survived on blackberries, prickly pears, worms, snakes, rats, spiders. Fulfilling the Moorish seer’s augury, he managed to work marvels. He stayed alive, chanced on three shipmates and convinced their Indian benefactors that he had the healing powers of a shaman. He proceeded to lay hands on the sick, recite the pater noster and miraculously make the dying walk again. In time, when he and his companions made their naked reentry into the Spanish stronghold of Mexico City, they found it more jarring than anything they had known in the wild. The Spaniards were full-bore into slave-hunting.

Varnum is wonderfully thorough, and although her prose never soars, her book is ever interesting for its inherent drama and fascinating detail. In the taut space of 300 pages, she manages to tell the whole tale: Cabeza de Vaca’s return to Spain, his famous report to the king and his subsequent, catastrophic captainship of Río de la Plata — the area that is now Paraguay. There, he was scoffed at for sympathizing with the indigenous people — especially the Guaraníes — and he was ultimately accused of and denounced for invented atrocities against them. It is a harrowing lesson in the power of propaganda and the age-old imperative of image management.

With resonant echoes of border crossings, human rights, culture shock and the prevailing rule of the big stick, “Álvar Núñez Cabeza de Vaca” offers much to any student of the Americas.

Marie Arana is the former editor in chief of Book World. Her recent biography of Simón Bolívar won the 2014 Los Angeles Times Book Award in that category.

ÁLVAR NÚÑEZ CABEZA DE VACA

American Trailblazer

By Robin Varnum

Univ. of Oklahoma. 368 pp. $26.95