A brief timeline:

1776 Some individual Americans declare independence from Britain, form a confederation in the name of 13 states (plural). 1783 Britain agrees that the US states (plural) are independent. 1789 States (plural) adopt new constitution changing the confederation to a federation. Slavery already a big sticking point (3/5 compromise + slave trade preserved for 20 years).
During this period it is unclear whether the US is like the EU is today (independent countries with some common government) or if it has become, in essence, a single country with the states being merely provinces.

1860 Anti-slavery party wins election. It becomes clear that slavery will eventually be made illegal throughout the US (though not necessarily for many years). 1860-61 Pro-slavery/southern states decide to leave the Union and form a new confederation among themselves so they can preserve the slave economy. 1861 US government declares the departure of the southern states illegal and sends the army to re-take them by force. 1865 US government wins the war and occupies the southern states. 1869 US Courts rule that states do not have the right to leave the Union without permission. 1864-1870 US Constitution amended to a) eliminate slavery, b) allow Congress to enforce equal rights within states regardless of race, and c) allow African-Americans to vote. Southern states re-admitted to the Union.

1877 US army occupation of the southern states ends. 1880 "The United States" becomes more often referred to in the singular, than the plural.

1954+ US Supreme Court rules many kinds of state-sponsored racial discrimination unconstitutional. 1964+ Congress enacts the Civil Rights Act and other laws that actually enforce equal rights and equal treatment of all races in major areas of the private sector economy.