Although most wars occurred during the dry season with its Underworld symbolism, the Maya looked to the gods for the exact time to launch a war, and the gods expressed their will by the movements of the stars. The Long Count dates on Classic Period monuments demonstrate that war repeatedly coincided with certain positions of the stars. Priests, consulting their books, could predict the time of eclipses and the first nighttime appearance of planets such as Venus and Mercury; such astronomical events were taken to represent the divine mandate to begin a war.

Venus, considered by the Maya to be the night sun, was the planet most associated with warfare in Mesoamerica. A hieroglyph called the star wars glyph occurred on dates that can be correlated 70 percent of the time to Venus when it is visible as the Evening Star. Warfare was avoided when Venus was invisible during superior conjunction, defined as a 90-day period between the last appearance of the Morning Star and the rise of the Evening Star. In many cases of territorial warfare, the Maya described their actions as a star-war event, which apparently sanctioned it as a divine mission. On these Venus war monuments, the Maya ruler often is depicted in a Tlaloc war costume (see figure 5.2, page 144).