(1) In WW1, either (a) no USA intervention, or (b) something made the Central Powers stronger. The war went on and on into the 1920's until nearly evrybody involved had weakened each other so badly that (x) the USA finally intervened and took over easily, or (y) the nations involved offered to become states of the USA to get access to postwar reconstruction help.
(2) In WW2 the Axis and the Allies both had nukes sooner and more of them, and in much of the war area USA had to take over a nuked wasteland because no effective local native governments could form.
In either case, USA inherits all the collapsed nations' colonies, except that India and Pakistan get separate independences more or less as OTL.
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Atlantica seems strange. I don't see how a small coastal sliver of South America can hold all those remote Atlantic islands. Unless it is based underwater, and events seem to be getting ASB-ish:-
Someone discovers nuclear reactors in the 19th century. He goes rogue loner, and then more or less as the fictional Captain Nemo. This spreads to more undersea bases and more nuclear submarines, and he and his successors get strong enough to grab a base on the South America coast given the confusion and weaknesses of the land powers of the time. Finally his knowledge of nukes leaks to the nations on land, and then as (2) above.