Which Aztec emperor ruled when Hernán Cortés arrived in 1519?

The Aztec emperor ruling when Hernán Cortés arrived in 1519 was Moctezuma II.

Moctezuma II, also rendered Motecuhzoma Xocoyotzin or Montezuma, was the tlatoani, or ruler, of the Mexica-led Aztec Empire. He had succeeded Ahuitzotl and was an experienced military and political leader before the Spanish expedition reached the Gulf Coast. Cortés landed in Mexico in 1519 and advanced inland with Spanish forces and Indigenous allies, including enemies of the Aztecs.

On November 8, 1519, Moctezuma received Cortés and his allies near Tenochtitlan and allowed them to enter the imperial capital. The encounter was not a simple meeting between two isolated sides: regional rivalries, diplomacy, military pressure, and Indigenous alliances all shaped the conquest. Tlaxcalan forces, for example, became crucial Spanish allies.

Moctezuma was later held under Spanish control and died in 1520. Spanish accounts claimed he was killed by his own people after addressing them, while other traditions say the Spaniards murdered him. His successor, Cuitláhuac, briefly led resistance before dying during the smallpox epidemic; Cuauhtémoc became the final Mexica ruler.