Thomas Jefferson significantly impacted American history as a Founding Father and the principal author of the Declaration of Independence. He championed the ideals of liberty, democracy, and individual rights, establishing the philosophical framework for the United States' government. Jefferson's role as the third President included the Louisiana Purchase, which doubled the nation’s size and expanded its influence. His advocacy for religious freedom and the separation of church and state set foundational precedents. Jefferson's complex legacy also involves his ownership of slaves, sparking ongoing debates about the contradictions in his vision for a free society.