In the thirties there were fascist parties in Latin Amerika. If the gained the power and established Regims like Mussolini or Hitler ... And then after Pearl Harbor when Hitler declares war on the USA they follow.
So the US are occupied with fighting Japan and South America, the can't send much help to Europe, and what arrives lands in England. So the Sowjetunion isn't supported so well. They still stop the Germans at Stalingrad, but they lack transport capacity to reconquer the lost terrain at the same speed as in OTL, and advance much slower.
In the meantime the USA is doing well against the South Americans and Japan. In November 1944 the fascist Regimes surrender. In process South America is totally occupied and de-nazified. Now the USA begins to send more forces across the atlantic (both to Brittain and the SU), the SU retakes Stalingrad and began with an offensiv. In England a huge bomber fleet is stationated which begins to attack the german cities. However Hitler has now VI and VII and the british cities had to suffer (altough less than the germans). As the Manhatten project is concluded, one bomb is droped on Hiroshima, the other on Cologne. While Japan surrenders as in in OTL Hitler fights on. To stop his industry the atomic destruction of the Ruhr area is planed and carried out. On June 6 the invasion takes place. When in 1946 the war is finally over in Europe, the continent is even more destroyed than in OTL. Moreover in Sout America there has been a rebuilding program (like the Marhall plan) for two years. And the side effects of the 8 atom bombs used against Germany become more and more visible. As the USA is fully occupied with rebuilding South America there is little aid left for Europe. In the following winter 1.5 Million people die. Sweden and Switzerland which both relied on trade with the rest of Europe suffer from an economical depression (Spain, Portugal and Ireland are able to trade with the USA). The other countries suffer even worse: their industry is destroyed, and famine breaks out. As a result refugees try to escape to Spain, Switzerland and Sweden and those countries close the borders. Needless to say, that under those circumstances the colonial empires collapse. Canada, Australia, New Zealand and South Africa are unwilling to support England, and the Commonwealth is disolved. Europe stays a backwater, where dictators like De Gaule arise. The countries which infrastructure and economy was largly spared - Spain, Portugal and Ireland - don't look to Europe, but over the atlantic, where under the lead of the USA a powerful economic and military Union arises. A normal european citizen in 1950 (and for the rest of the century) wants only to emigrate to America (or Australia or South Africa), to escape poverty, suppression by the own regime, and fear of a Sowjet invasion. Fortunatly for them the thriving economies of Argentinia, Chile and Brasil have an enormous need for workers. But those who leave are the wealthy and well educated, and so the emmigration drives the rest of Europe even deeper in the misery.