The Declaration of Independence was composed by Thomas Jefferson to declare the American colonies' separation from British rule and to assert the principle of equality among men, stating that all have unalienable rights to life, liberty, and the pursuit of happiness. Written in 1776, it served three main purposes: outlining reasons for separation from King George III, establishing a theory of government based on democratic principles, and formally declaring the existence of war. The document marked the beginning of a new nation and the evolution of concepts related to rights and governance.