Maya observed the sky and calendars to predict solar and lunar eclipses, the cycles of the planet Venus, and the movements of the constellations.
The best choice would be the Nanjing Massacre led to the increased tension between the United States and Japan in the year 1937. This is because didn't like what the Japanese did in China and in other parts of Asia. The killings they made to many are not really favorable to the U.S.
