The following items have threaten the unity of USA at some point in history: powerful European nations (<em>such as France when ruled by Napoleon Bonaparte and controlling the state of Louisiana</em>); the war of 1812; Great Britain when impacting American Shipping and forcing American seamen into British service; Abolitionism; The Civil War; Nationalism and Liberalism; slavery and xenophobia.
The Olmec's developed one of the earliest known civilization in the Americas. They settled along the Gulf of Mexico around 1200BCE. Because agriculture was very successful. that led to growth of population.
To protect the country from other powerful naval forces. text me ;)
