One of the native tribes that allied with the Conquistadores against the Aztecs if I remember correctly. In fact, I vaguely recall (from Explorers of the New World) that they made arrows for Cortez’s men that proved superior to the ones they’d brought with them from Spain.

Yes, Tetzcoco played a crucial role during the last phase of the war. However, they were originally the main alliance of Tenochtitlan. Cacama was nephew of Motecuhzoma so they were closely related. However, Cortez killed both of them right before the 'Noche Triste' (1520). It was 1521 when Tetzcoco joined Cortez by their new king Ixtlilxochitl. I recommend Matthew Restall's "When Montezuma Met Cortés" for more in-depth explanation of the war.

Also, the term 'tribe' is incorrect since Tetzcoco was one of the main 'city' of the Triple Alliance (so-called 'Aztec Empire'). Tetzcoco's population was at least 25,000 and they had temples, government officials, city districts, school, library, vassal states etc. So the term 'city-state (altepetl)' is more correct. Actually, most of the Mesoamerican polities were city-states at the time of Cortez's arrival.

I see.