As the US became developed the country abandoned the isolationist policy and started to get involved in a series of international issues. Quickly the American government realized that if the US started to support the European Colonies, especially those in Latin America, and helped them to gain independence from Europe, there would be a big sphere of influence on the American continent. This is called the big stick diplomacy first developed by Theodore Roosevelt. With that the US became more and more relevant internationally.
The orders were to take everything that they can, and burn and ruin everything that they can't take. The idea was to destroy infrastructure completely so as to weaken the southern population and the southern support for the war. They would burn farms and pillage everything leaving nothing standing.
