Delete if this is sidetracking. The real answer is probably that the United States emerged as a “great power” in the late nineteenth century and a “superpower” after World War II. Someone else can explain that better than me though.

But just a fun addition on this topic re when it became “obvious”—people predicted the US would become a mighty empire all the way back in the mid-eighteenth century based on the then-British colonies’ explosive demographic and territorial growth. One demographer in something like the 1750s calculated when the American population would outstrip that of the mother country’s (I believe he guessed the late 1860s or the 1870s) and got it pretty close.

People thought that the center of gravity of the British Empire thus might be destined to shift to America, speculating that, for example, in a century the monarchy would move permanently across the Atlantic. More fabulously, Brits occasionally imagined a future where London became an ancient Rome-style ruin being gawked at by tourists from New York and Virginia.

This sort of futurism had real implications though. From very early on many American rebel leaders had a strong sense of what would eventually be termed manifest destiny—believing if they could just throw off the might of the mother country in the present moment a great empire would eventually belong to them and their descendants. On the other hand, British policymakers were deeply concerned that American strength would allow the colonists to break the bonds of empire and strategized about restricting western expansion or alternately restricting urban growth, imposing garrisons, restricting immigration to America etc as means of checking this process.