The United States has been in decline as long as anyone has been alive. Longer than that in fact. The fall predates the Obama presidency, the Civil Rights era, the Reconstruction period, and even the Civil War. We have to go back further.
America's status as an empire is also longer than history typically claims (thus its decline from empire is longer). Court historians would say imperialism began in the United States with the Spanish-American War (1898), when the government acquired formal overseas colonies which were never to be integrated as states but governed by appointment. Perhaps those looking further back might argue that the acquisition of Alaska (1867), the Northern administration of the South (1863-1877), the Mexican-American War (1846-1848), or the Louisiana Purchase (1803) were the start of an American Empire, and while they would be warmer they would still be off by some margin.
The Thirteen Colonies by the 1700s were colonies in name only. United they formed an empire with 1000 miles of coast and 430,000 square miles of territory. For comparison the area of the modern United Kingdom is 93,600 square miles. Today the area United States has grown to 3,800,000 square miles, making it larger than the Roman Empire under Trajan (1,930,511 square miles).
Size is not descriptive of spirit however, because history's greatest Aryan empire has never truly conceived of its purpose in a consistently correct way, to its great misfortune. A state can both increase and degenerate, which is the story of American history. While it may seem counterintuitive it has been transpiring for nearly two centuries. Our race has expanded greatly, but our ability to manage that expansion has atrophied and so we have had our destiny hijacked.
Yet we must remember that we did this to ourselves first. Mosquitoes require a swamp to breed.
I would posit that the first event in the decline and fall of the American Empire was:
The Mexican-American War (1846-1848) - The failure to completely annex a defeated country during a high tide of racial expansion and folkwandering marks our starting point. Because of regional factionalism between the North and South, only Texas and the less populated areas of northern Mexico were acquired. Since the anti-expansionists won, we should consider their winning arguments (however petty and small-minded they may have been in the grand scheme of Aryan colonial expansion). These were that expansion would
1.) Increase the political power of the slaveholder states relative to the non-slaveholder states because it was expected that slavery would be spread into the Mexican territories,
2.) Bring large numbers of non-Europeans into the United States, and
3.) Bring large numbers of Catholics into the United States.
For starters, it had already been legally established under the Missouri Compromise (1820) that slavery was allowed south of the 36° 30' parallel, and so opposition was really a Northern jealousy over their own lack of perceived benefit from expansion. (Never mind that almost all American shipping was done through Northern ports, and ships carried Southern exports or products made with Southern raw materials). Thus arguments against Mexican expansion--expansion into the tropics--became by default one in the same with arguments against slavery and the Southern caste society. In the North because there was at that time no substantial non-Aryan population, the increasingly popular conception of slavery was not one of managing an interracial society but of inflicting evil on blacks and Whites through what was seen as an immoral institution. (Never mind that the so-admired and "virtuous" Greeks and Romans who inspired the American Republic were slaveholders). And so the North did not want slavery to be expanded into Mexico as that would mean at least a dozen new slaveholder states and a permanent Southern orientation of the United States. The Golden Circle--the unification of the New World's plantation societies under one banner--would have reduced New England and her Midwestern scions to a fiefdom. So the North prevailed against the expansion of White rule into Mexico because that would mean a government dominated by planters.
The other two arguments at first may seem more racially and culturally sensible. Annexing all of Mexico would have meant more non-Europeans. But the entire territory of what is today United States had 250 years before the Mexican-American War contained nearly no Europeans, and plenty of other peoples. Their presence has simply never been an obstacle to our expansion. More importantly, there was already existent in 19th century Mexico a mostly Aryan ruling caste, and that combined with European and interstate American migration along with birthrates would have significantly Aryanized the would-be territories over time.
On the issue of Catholicism, it seemed to have not stopped the British from contemporarily ruling Ireland (under a government where freedom of religion was not enshrined no less) so why should it have proved so greater a problem for the United States ruling Mexico? The irony should not be lost that today the plurality religion in New England is Catholicism, and hence their complaints were all for naught since they allowed its spread to occur anyway.
To summarize, an opportunity to expand the lands available for rule and settlement of the Aryan race was made available to the Americans in the war against Mexico, and the Americans declined to acquire the maximum possible lands because of political infighting. An opportunity to expand our best bred and most vigorous pioneering peoples was turned down. This would not be the last time such self-sabotage was enacted. But it kicked off a long decline for certain in an empire where for two generations earlier settlers had relentlessly driven the British, French, Spanish, and Indians out of what went from the border of the United States to its geographic center.
Today we live with the bitter, bitter irony that the United States is becoming a mostly black and brown country, but on New England's terms of racial equality and miscegenation rather than the South's, and with much less territory. Had the South prevailed and the United States became a significantly blacker and browner country, but on the terms of Aryan supremacy, we would live in a very different timeline indeed.