The country was founded in a revolt of colonists from Great Britain, ca 1770 CE. The new country's government and fundamental laws were designed taking into account Enlightenment ideals of the preceding several decades. They attempted to prevent just those abuses of power resentment of which had begun the revolt. As well, those provisions attempted to protect the individual as against the interests of the government. Its approach was influential, though honored too often in the breach, throughout the world over the next few hundred years.
Western Europe had, collectively, adopted many of the ideals of the United States by the time the expedition to Manticore was organized, largely from Western Europe and areas of Earth influenced by Western Europe. The core values of the Star Kingdom of Manticore (and of the official Solarian League, for that matter) owed much to the political philosophy tradition exemplified and in many respects pioneered by the United States. Examples include effective freedom of the press though with reduced tolerance for intrusive rudeness in the gathering of news, a largely government free commercial sector, general government protection for minorities, rule of law, personal choice of religion/political positions/profession/residence/mate(s)/sexual orientation/etc, ...
Being from Western Europe, the Manticoran settlers were most comfortable with a Parliamentary style of government rather than the United States' model (ie, fixed terms for the executive branch) which predated the rise of political parties and did not cope well with them. (UHH, HH1)
During the 19th Century CE, the United States fought a brief War with England. Two naval battles (ie, wet sailing navy), among others, were long remembered from that War. The Battle of Lake Champlain and the Battle of Lake Erie both happened on inland lakes, had both tactical and strategic importance for the War, and both became legendary among Naval historians and enthusiasts. More famous engagements of that War were fought at sea by the USS Constitution; it survived the War still afloat. It was retained on active, though limited, service and remained in full commission for what was intended to be perpetuity; it left the dock only once a year to sail into Boston (Massachusetts) harbor, so it could turn around and again dock, but in reversed fashion. It was for the duration of the United States the oldest commissioned vessel in the US Navy, and one of the oldest in the entire world. This tradition was continued thousands of years later for at least one RMN starship, HMS Unconquered.
Later in that century, a small squadron of the United States Navy appeared in Tokyo Bay, startling the hitherto successfully isolationist Japanese, and initiating the entry of the Empire of Japan into the world community. Commodore Matthew Perry began trade and diplomatic relations between the United States and Japan.
The Church of Humanity Unchained originated in the USA as a protest in both religious and cultural terms against prevailing social and religious norms; a contingent also left Earth soon after the Diaspora began, but for different reasons than did the Manticoran expedition. they wished to establish religious and social purity in a corrupt and corrupting dominant culture. Its tenets, in somewhat matured form, and some descendants remained important in some parts of human space into Honor Harrington's time and beyond. (HH2)