The September 11 attacks on the United States by the pan-Islamist militant organization al-Qaeda led to the war on terror and subsequent military interventions in Afghanistan and Iraq.
The United States Constitution went into effect, creating a federal republic governed by three separate branches and ensuring a system of checks and balances.
The Treaty of Paris was signed, recognizing American sovereignty and marking the end of the American Revolutionary War. The treaty established the boundaries of the United States, stretching west to the Mississippi River.
The Second Continental Congress adopted the Declaration of Independence, formally declaring the 13 American colonies' independence from Great Britain. This event marked the birth of the United States of America as an independent nation.
The first known public usage of the phrase 'United States of America' appeared in an anonymous essay published in the Williamsburg newspaper, The Virginia Gazette.
The establishment of Spanish Florida, the first European colony in what is now the continental United States. This marked the beginning of European colonization in the region.