The united states became a society of consumers in the late nineteenth century. what was the key factor that made that possible

Industrial Revolution is the correct answer.

Before the Industrial Revolution, most goods were either homemade or bought from local stores. During the rise of industrialization in the United States, Americans had great access to consumer goods. Companies were investing lots of money on advertising and offering shoppers more convenient ways to place orders. Since big companies were producing a big variety of products and fighting over customers, the prices were lowered which made it possible for even the lower classes to have the opportunity to make purchases.

these civilizations were located around fertile rivers, where they could produce an abundance of crops

declare war, appoint military officers, sign treaties, make alliances