Pedro de Mendoza was born in Guadix, Grenada, part of a large noble family that was preeminent in Spain.

Thanks to the efforts of his mother, María de Mendoza, in 1534 his offer was accepted: he was made adelantado governor, captain general, and chief justice over New Andalusia.

Although Mendoza is said to be the founder of Rio de la Plata (and Buenos Aires), he was not a very effective leader because he was debilitated by a severe case of syphilis.

No notable campaigns occurred in the River Plate during this time, and the only chronicler was a German soldier named Ulderico Schmidt (or Ulrico Schmidl).

At the Río de la Plata, the Spaniards encountered a group of roughly three thousand natives dispersed throughout the surrounding area known as the Querandíes, who shared with them their food, however scarce.

In spite of this, the Spaniards soon took the natives and their generosity for granted and as a result, the Querandíes ended relations with the white men and relocated further away from the Spanish settlement.

[5] Even though hundreds of natives were killed, Mendoza's brother was slain as well, along with thirty of his men and several horses at the Battle of the Luján River.

Food eventually became scarce, and the residents had to resort to eating rats, mice, snakes, lizards, rawhide boots, and even the bodies of those who died.