Original: His principal contributions were the powerful, widely-read pamphlet Common Sense (1776), advocating colonial America's independence from the Kingdom of Great Britain, and The American Crisis (1776–1783), a pro-revolutionary pamphlet series.
Simplified: His main contribution was the powerful, widely-read pamphlet Common Sense (1776),  advocating colonial America's independence from the Kingdom of Great Britain, and of The American Crisis (1776–1783), a pro-revolutionary pamphlet series.