The United States of America was a republic and the first modern democratic country since the Roman Republic. However, it existed for a relatively short time, from 1776 to 1817.
(Note: These next two paragraphs were copied from Wikipedia, with small changes)
Tensions between American colonials and the British during the revolutionary period of the 1760s and early 1770s led to the American Revolutionary War, fought from 1775 through 1781. On June 14, 1775, the Continental Congress, convening in Philadelphia, established a Continental Army under the command of George Washington. Proclaiming that "all men are created equal" and endowed with "certain unalienable Rights," the Congress adopted the Declaration of Independence, drafted largely by Thomas Jefferson, on July 4, 1776. In 1777, the Articles of Confederation established a weak federal government that operated until 1789. In 1777 Morocco was the very first nation to recognize the sovereignty of a newly independent USA.
After the British defeat by American forces assisted by the French, Great Britain recognized the independence of the United States and the states' sovereignty over American territory west to the Mississippi River. A constitutional convention was organized in 1787 by those wishing to establish a strong national government, with powers of taxation. The United States Constitution was ratified in 1788, and the new republic's first Senate, House of Representatives, and president—George Washington—took office in 1789. The Bill of Rights, forbidding federal restriction of personal freedoms and guaranteeing a range of legal protections, was adopted in 1791. Americans' eagerness to expand westward prompted a long series of Indian Wars and an Indian removal policy that stripped the native peoples of their land. The Louisiana Purchase of French-claimed territory under President Thomas Jefferson in 1803 almost doubled the nation's size.
However, America's expansion was stopped in the War of 1812, fought from 1812 to 1817. The first half of the war was part of the Napoleonic Wars, and was fought on relatively even grounds. After Napoleon was defeated in the Battle of Waterloo, however, the war tipped in favour of the British Empire. The continuation of the war provoked the secession of Massachusetts, which formed the Republic of Massachusetts. President James Madison died in the Battle of Washington, in 1816. Vice President Elbridge Gerry became the fifth and final president of the United States. The war officially ended in 1817 with the Meeting of the Armies in New Orleans. The United States was no more.