Andean Abyss
As a result of a speech, the two sides in the rapidly expanding South American war gained their own sobriquets. After a Brazilian speech alleging the creation of a Monteveido-Bueno Aires axis against Brazilian interests, the Americans, Argentinians, and Peruvo-Bolivians quickly became known as the Axis Powers. In contrast, the Brazil-Paraguay-Chile alliance simply became known as the Allied Powers, largely because they couldn't figure a better name even as the second year of the conflict started.
Although Chile and Argentina had some of the longest borders of any nation on Earth, the border was largely non-passable, being comprised primarily of the Andes Mountains. The war would be primarily waged in the notorious inhospitable Patagonia Desert, which worsened the supply issues of both sides. However, one side had a clear advantage. The Chilean Navy had seized much of Tierra del Fuego early in the war, and the majority of Patagonian laborers (living primarily in the wooded west and various islands) were actually of Chilean origin. Argentine army officers were given orders to imprison or kill all Chilean laborers, which immediately proved to be a largely impossible task. Chilean laborers, fearing for their safety, stole weapons and formed small bands of fighters. in these forested areas With limited naval supply, operating in a desert, and facing a largely hostile population, the Argentine Army largely proved hapless in Patagonia, being unable to make their own offensives. That being said, they were able to hold strategic positions in the desert by building significant earthworks, and the difficulty of supplying an army in the Patagonia made it difficult for the Chilean Army to actually dislodge them. Although Chilean forces seized several forested areas, they were unable to overrun Patagonia as desired. The war in Patagonia quickly devolved into endless small-unit skirmishes in the desert. The Patagonian Front notably saw the first serious military use of aircraft. The 1906 Wright Flier, commissioned by the US Army and lent out to the Argentine Army, was the first aircraft used for military purposes. In practice, it was essentially only used as a scout aircraft, giving Argentine troops a significant advantage in many of these skirmishes. Although the first year was filled with successful Chilean offensives in the forests of the western Patagonia, the second year was seen as largely a disappointment, losing small-scale firefights.
Farther north in the Andes, the Chilean offensive also met partial success but great frustration. After the Brazilian Navy returned from Europe. Chilean forces managed to achieve local naval superiority in the Atacama Desert, which allowed Chilean forces to interdict any supply going to beleaguered Peruvo-Bolivian garrisons. Surely but slowly, the Chileans slowly simply starved out these garrisons. Determined Peruvo-Bolivian garrisons, essentially trained and armed by the United States, could repel any direct assault, so the decision was made to resort to starvation and drought. The first year essentially saw Chilean forces take horrible losses trying to push against garrisons. The second year of the war saw much more success for the Chileans. The real dilemma presented to the Chileans was whether to take a further gambit. Some wanted to leverage their local naval superiority into an amphibious landing to target and occupy Lima (the largest city in Peru-Bolivia), hopefully bringing the war to an end. Another option was to simply hold in the Atacama, try to seize the Peruvo-Bolivian capital of Tacna (next to the Atacama along the coasts) and then try to outlast the Axis powers. The latter seemed to have an advantage in internal debates, since it was widely understood that the United States would be sending a fleet from its Pacific Coast, which would threaten Chilean gains.
However, the decision would soon be influenced by an unexpected event. The "Night of the Bayonets" severely damaged morale in Peru-Bolivia - even though the President of the United States personally made sure that the officer in charge was court-martialed, it sparked widespread popular anti-Americanism. As the power closest aligned to the United States, and in fact as a nation whose entire existence was propped up by American funds, Peruvians and Bolivians were deeply concerned by the Montevideo Massacre, wondering if they would be next. The President of Ecuador, Eloy Alfaro, had earlier taken power in a coup, overthrowing Ecuador's conservatives Catholic establishment. As a secularist and militarist, Alfaro had built the Ecuadoran Army into a fairly impressive, albeit small, fighting force. Moreover, the Allied Powers had begged for Ecuadorian intervention during the entire war. Due to a complex history of colonial borders, Peru's Loreto region in the Amazon was seen by the Ecuadorans as occupied territory. With large rubber and timber deposits, the region became increasingly economically lucrative. Finally, it bordered Brazil, which thus had interesting strategic implications.
In the aftermath of the Argentine-American conquest of Uruguay, the Ecuadorans found to their surprise a large number of Peruvians offer to defect. With the opportunity having essentially fallen into their lap, Ecuadoran soldiers urged action, and fearing violence from his own soldiers, Alfaro made his move. Ecuador entered the Second World War as its army flooded across the border, quickly seizing towns largely without a fight, and replacing the Peruvo-Bolivian flag with their own. The Ecuadorans, having fortified their coastal border with Peru, had hoped that they could make the seizure of land essentially fait accompli. After all, facing a multi-front war, perhaps Peru-Bolivia would simply just accept the loss of those territories as opposed to undergoing a hideous war to take them back (in poorly supplied, heavily fortified regions).
Indeed, Tacna seemed obliged to simply let the territories go. The capital of Tacna was under direct Chilean threat and morale was extremely shakey. Almost nobody in actual Peru or Bolivia had originally supported the restoration of Peru-Bolivia - it had been largely an American initiative and even then, few now trusted the Americans. Moreover, the Peruvo-Bolivian offensive into the Chaco Desert had largely collapsed in the face of fierce, and better-trained Paraguayan resistance. Patriotic Paraguayan soldiers had a massive morale advantage against what were largely poorly trained and equipped Quechua and Aymara peasant conscripts, who were led primarily by primarily white Bolivian and American officers, some who were quite racist. This large morale disparity was further exacerbated by the Night of the Bayonet, further frustrating Bolivian-American officers.
However, the decision was made for Peru-Bolivia. The US Congress almost immediately voted upon and passed a declaration of war against Ecuador, horrifying Peruvo-Bolivian leadership, who then had no choice but to comply and follow with their own declaration of war. Although the first year of World War II in South America saw various Axis triumphs (chiefly the occupation of most of Brazil's population centers and the integration of Uruguay into the Axis), the second year was largely seen as an Allied victory, with the Chileans making gains in both Patagonia and the Atacama, while Paraguayan forces retained control of most of Misiones and completely routed Peruvo-Bolivian troops in the Chaco. Going into the next year, with a presidential election upcoming in the United States, the Beveridge Administration significantly accelerated plans for a large counter-offensive across the Pacific coast of South America, eager to put the war back on track for the Axis with the nearly unparalleled industrial and economic strength of the United States. The Americans figured that a negotiated peace would be signed shortly after the occupation of Brazil...but that simply was not to be, leaving the second year filled with missed American opportunities. Now the goal was to quickly end the war with overwhelming force.