These included Kilwa, Sofala, Mombasa, Malindi, and others. The city-states traded with inland kingdoms like Great Zimbabwe to obtain gold, ivory, and iron. These materials were then sold to places like India, Southeast Asia, and China. These were Africa's exports in the Indian Ocean Trade
The Revolutionary War with Britain after British colonists were already in the US, sent by the British. Basically, they were being taxed on stuff like Molasses and Tea. They didn't want taxation from a country in which they had no representation in government. This was the reason for the Boston Tea Party. Then of course they formed their own independent country once they were free from Britain.
The AG industry was able to provide way bigger amounts of food. With higher amounts and productivity gave farmers a easier time to keep up with the growing population around the world. more people means higher demand for food in larger quantities.
