Thomas Paine wrote The American Crisis--a 16-pamphlet series published primarily during the early years of the American Revolution--to bolster support for the American colonists' efforts to gain independence from Great Britain. His intended audience was primarily American patriots.
When "Crisis no. 1" was published in December 1776, patriot morale was low, and there was much uncertainty about whether the Americans had the strength and fortitude to defeat the mighty army of Great Britain. Paine sought to increase their morale; thus he used simple language which commoners could understand. "Crisis no. 1" was read to the Continental Army prior to the Battle of Trenton and helped to rally the troops.
While disheartened patriots were his primary audience, Paine also hoped this pamphlet would convince people still loyal to Britain to turn and fight for the Americans.