How did the United States begin?

This question depends on how you view the nation's origins. The English colonists lived in what we would later call the United States. During the Revolutionary War the colonists were united by the common cause of defeating Britain and gaining their independence. The colonies were linked under the Articles of Confederation but this was a very loose connection as the states had to agree to nearly everything unanimously, which proved impossible and unworkable. The United States ratified the US Constitution in 1787 and this made the states more unified as the states were locked into a federal system.
The Constitution was challenged as well as states questioned whether or not they had the right to leave the Union. This final test was answered in 1865 as the rebellious Confederate states surrendered and began the long road back into the Union. One can date the United States's origin from any of the above time periods—it all depends on one's definition of "united."