The United States of America was established in July 1776 when the first 13 states – then known as individual colonies – agreed to the United States Declaration of Independence. The first state to join the union was Delaware, which ratified the 1787 Constitution (which created the federal union of states that exists today) on December 7, 1787. Pennsylvania closely followed it on December 7, 1787, and New Jersey on December 18, 1787. A further ten states along the eastern seaboard made up the original 13 states of the United States of America – Rhode Island was the last of these to join in May 1790.
Over the next 20 years, a further 11 states joined the union, including Ohio (1803), Louisiana (1812), and Maine (1820). There was then a gap of 15 years before the next state joined – this was Arkansas in June 1836. Two of today's most populous states, Texas and California, joined in 1845 and 1850, respectively. By the end of the 19th century, the majority of the states that make up the current union had joined. Utah was the last 19th-century state to join in January 1896.
Throughout the 20th century, only a further five states would join the union. These were Oklahoma in November 1907, New Mexico in January 1912, and Arizona in February 1912, while Alaska and Hawaii – the only two non-continental states – joined as recently as 1959, taking the total number of states in the union to 50. Today, Americans are citizens of both the state in which they live as well as the Federal Republic of the United States. Interestingly, Massachusetts, Pennsylvania, Virginia, and Kentucky continue to use the term 'commonwealth' rather than state in their full official names.