I stand in to remark that it is actually quite easy to give the USA a much better head start and make it much more successful than OTL, my signature TL is among other things, dedicated to prove that principle.
Just give the USA a few very useful nudges at the start:
A) make Canada join the USA in the ARW: this greately enlarges their resources, expands their economic base, and hugely improves their strategic standing vs. a weakened British Empire in any future confrontation.
B) make early American culture more accepting and assimilationist of non-WASP people: this makes them have no qualms about annexing huge swaths of Latin Americans and giving them equal rights as citizens, as opportunities arise again and again during the 19th century: the Mexican War and ATL intervention in the Spanish American Wars of independence are two obvious ones, the Spanish-American War is another one. History proves that America has potential for cultural and political assimilation of new citizens that is only matched by Rome.
C) nudge American culture towards biparisan support of expansionism, militarism, and economic interventionism: this shall make the country be settled, industrialize, become militarly strong more quickly (a large part of US military weakness up to the ACW and WWI was self-induced, they could have built much bigger and better armed forces with their economic base), which creates more potential for greater expansion and more opportunity to put the extra land, people, and resources you annexed to good use.
This combination may easily yield a USA that by 1900 owns all of the Americas and choice bits of the Pacific, creating a fitting power base to crush and assimilate the British Empire and/or Europe in a couple of World Wars, provided that Europe does not unite to match super-America and/or China and India do not make their post-colonial take off much earlier.
USAO America could have easily gone on a successful WC. The TL is instead steering towards a 1984-like tripolar setup only because I purposefully let butterflies groom a parallel united Euro-Russia match, but it could have gone a different way.
IMO the British Empire never had the full potential for WC expansion and assimilation that America had, if you nudge it to be more expansionist and less exclusionary and snuff out Canada, it lost its true opportunity for WC when it lost America.