Irrigation of crops led to salinization of the soil, Sumer was located on a flat plain that provided no natural barriers to invasion, and the land became too swampy to grow adequate crops.
Judging from your other questions, I would guess the Aztecs, because they had an extremely advanced civilization relative to their time period. However, you might want a second opinion to verify this. Hope this helps.
China it was modeled after
He got to become a government
India got freedom on August 14th, 1947
