With the beginning of the War of 1812, the economy of the 30-year old United States of America began to falter. Both France and Britain did not want us to trade with the other. New England, in particular, was feeling the worst part of the economic pains. The Hartford Convention began on December 15, 1814 and ended with the announcement from the delegates that New England would seek independence no matter what. The date was January 5, 1815, New English Independence Day.
The rest of the nation was in a fight against the British, and with New England opposing the United States the war ended very quickly. New England made a deal with England, supporting it militarily. With much of the navy in New England, many naval battles were fought. Also, with New England breaking away, it opened a new front. The war ended on February 1, 1815 with the signing of the Treaty of Ghent.
With the War of 1812 finished, the United States was barred from trading with any other nation except Britain and from expanding northward. The economic problems quickly ensued and the United States was in another civil war in 1835 and 1859. It was involved in a short war with Mexico in 1846, which it lost, preventing the annexation of Texas. By 1870, the United States had lost New England and the Southeastern states.
New England itself enjoyed great prosperity after the War of 1812. It traded with the British Empire, and it built up its navy. Even though Boston was the largest city, it was seen as a memory of the United States, so it was not made the capital. Hartford of the state of Connecticut was made the capital. By 1900, New England became the richest nation in the western hemisphere and one of the richest in the world. World War I saw the involvement of New England on the side of their ally, Britain. The Confederate States and California joined New England. World War II also saw New England on the side of the British once more. The Confederacy fought alongside the Nazis, though. With the constantly neutral United States between them, each nation took over parts of the United States to fight each other. World War II ended in 1948 with a win for the Allies, though.
The Modern Age saw the decline of New England as a traditional industrial power. New England's economy muddled through several tough decades, before re-inventing itself as a world leader in financial services, high tech engineering and software design, and is currently recognized around the globe for it's health care system.