From TWC Wiki
The Tlaxlans are relatively small in the beginning but have a large army at their disposal. They start at war with the Aztec Empire.Their units are similar to the Aztecs. Early on the tlaxcalans have a difficult choice to make: to ally or not with Spain. Altough allying with Spain may be good to the war with the Aztecs, Spain will probally become too strong for the Tlaxcalan player.To make matters worse the Aztecs are very numbeless in terms of armies and they border your maps and Spain also bordes the Tlaxcalans meaning you are in the centre of the two greatest powers in America. However if you are wise you can swallow the Aztec cities for yourself you can prevent Spain from becoming too powerful.