Hey! South American Anthropologist Powers Activated! It was good that all SA opposed Nazism(in a given moment). However, the reason why they did choose that, was not altruistically motivated. I will focus on this : so did a declaration of war in 1945 really change anything?

TL;DR:

1. South America always has been the playground of US geopolitical ambitions. The war was over, therefore it was a good move to play safe with the the Giant of the North. 2) Many goverments in the region flirted with the totalitarian regimes during the last decades, again it was a good move to play safe to the US. 3) German and Italians inmigrants created sucessfull business and comunities during the first half of the century, the war was a pefect excuse to assimilate and nationalize some busssines within their hands. 4) Brazil was actively courted and forced to join the war seriously(in addition to all the the other causes).

LONG BLOCK OF TEXT

Until 1942 all SA(SouthAmerican) countries had heavily enforced neutrality policies about anything that happened in Europe. But stuff happened:

1. South America commodity-based economies suffered a lot during both wars. Many goverments tried to keep trade open to both sides of the war at the same times(See Lefebvre, 2007). Therefore, until the goverments did not see that war was finally over they did not choose sides. Obviously, they picked the winning side: U.S.A. Concurrently, the Americans started huge programs to minimize axis influence in the goverments (Helleiner, 2014). Those programs also were the prologue to the cold war batlefield that South America was about to become. The Lame South American equivalent of the EU, the OEA, started with the Pan American States Conference in Rio, on 28 January 1942. Where all SA countries severed their diplo ties with the Axis
2. HOWEVER, there is the problem that many people in regional politics in the were personally ambivalent, at least, to European Totalitarisms. Laureano Gomez, Colombian main opposition leader, did copy the bad art museums and was fan of Franco state policies (Rivera, 2003). In Chile in 1938 there was an attemp of Nazi-backed coup in 1938 (Silva, 1938). Argentina civil leaders were pro-allies, but the army insisted in not breaking the neutrality policy(and many of them were pro-axis). Then, in 1943 the Army did a coup d'état, and they choose a pro-allies(Rawson) as the military junta leader to avoid US intervention. Therefore, with the war ending many of those leaders had to to something to congratiate them to the US goverment (More on that in Lefebvre, 2007).Bonus: Laureano Gomez did became president eventually. And he was, basically, extorted because of his Nazi-light past, and he ended sending Colombian troops to Korea to redeem himself with the US.
3. The second commercial airline to operate in the world was SCADTA, the colombo-german airline. The German, Japanese, Italian inmigrant communities created many sucesfull business. That those big-state leaders of the second paragraph did not lose the opportunity to force them into more 'traditional' wealth-owners (Friedman, 2000). In one hand, also US industrialists found a perfect excuse to erode local competition for their own industries(Callcot, 2014) The SCADTA case, where many of their routes were handed to PAN-AM). In the other hand, it was free money for many cash strapped goverments of the region (Friedman, 2003) . Bonus: SCADTA stills operates today under other name, AVIANCA that is the biggest carrier of the Region, no german planes though.
4. Brazil was the only country that actually sent troops (therefore the only one that did something in the war): The Cobras Fumantes, a division kind like the Flying Tigers of the US in China (Coutinho, 2017) . In addition to all the stuff that have mentioned, Roosevelt actively pressured the Vargas goverment to open Brazil isolationism, and which best way to do it than participating in a global show-down. Roosevelt went personally to Brazil, and made them compromise to join their side inside an USS : The Potenji River Conference. Inside Brazil there were massive doubts and resistence to participate in the war, but many expected Brazil to start to flexing their muscles in the world concert(take in account that Brazil is a huge country with huge population) .

Bonus: That division have a sick emblem ( A smoking cobra )

PD: English is not my first language, I do apologize for that kind of mistakes. Won't use the <American> noun, though.

PD2: I do apologize, also, for the style. Edit: More Style issues

PD3: Latin American RIGHT-LEFT cultural wars are exciting, awful and still generates a lot of controversy. So you may find people that say that their leader was the paramount of liberty since the Pusch Attemp. ¿Pero pues que se le hará? Son los datos y hay que darlos!

References

Friedman, M. P. (2003). Nazis and Good Neighbors: The United States Campaign against the Germans of Latin America in World War II. Cambridge University Press.

Friedman, M. P. (2000). Private memory, public records, and contested terrain: Weighing oral testimony in the deportation of Germans from Latin America during World War II. The Oral History Review, 27(1), 1-15.

Silva, J. P. (2010). La matanza del Seguro Obrero (5 de septiembre de 1938). By Marcus Klein. Santiago: Globo Editores, 2008. Pp. 200. Illustrations. Documents. Bibliography. The Americas, 67(1), 134-136.

Rivera, H. E. P. (2003). Acerca del nacionalismo católico de Laureano Gómez 1930-1946. Revista Colombiana de Sociología, (20), 31-40.

Helleiner, E. (2014). Forgotten foundations of Bretton Woods: International development and the making of the postwar order. Cornell University Press.

Lefebvre, A., Lauderbaugh, G., Masterson, D. M., Mount, G., & Perez, O. J. (2007). Latin America During World War II. Rowman & Littlefield.

Callcott, W. H. (2014). The Western Hemisphere: Its Influence on United States Policies to the End of World War II. University of Texas Press

Coutinho, E. M. R. (2017). A cobra vai fumar: a memória a partir dos objetos da Força Expedicionária Brasileira (Belo Horizonte, Brasília e Campinas).