In 941, during WWII, Ecuador and Peru went to war with each other. It had nothing to do with the conflict between the Axis and the Allies, rather it was born of a long-standing border dispute between the two countries.
The dispute stemmed from the wars of independence period of South America, with Peru and Greater Columbia (which is what Ecuador was to branch off from) as well as Portuguese Brazil being in the mix to the east. Several wars were fought in the 19th century, and by the 1920s an uneasy truce prevailed between Ecuador and Peru.
In the early 20th century Peru's army greatly enlarged and improved, with modern equipment including artillery, tanks, and aircraft. Ecuador relied on diplomacy to fend them off, but with the U.S. being increasingly focused on the wars in Europe and Asia, South American affairs became lower priority and the crisis deepened.
Still the Ecuadorians thought they had a surplus of fighting spirit in their troops over the Peruvians. From Ecuador and Peru, 1941:
With all this [Peru’s significant numerical superiority], I should affirm that Ecuador is superior to Peru in terms of race; the Ecuadorians have a warring nature, while the Peruvians are pacifists. Peruvian officers are good because regular contingents of young men study in military schools abroad. Consequently, the Peruvian General Staff is efficient and well-trained, but the troops are inferior to those of Ecuador.
That turned out not to be the case and based on that misjudgment Ecuador began reinforcing troops in the disputed territories. This led to heavy skirmishing during July 1941 which led to a full Peruvian invasion of Ecuador. In a South American version of a blitzkrieg, which even included parachuters, it was over quickly. By August the Ecuadorian resistance collapsed.
They signed a truce of sorts, although Peru massively violated it at least twice, and then they settled down with the border dispute still simmering. Two more small wars broke out in 1981 and 1995, but the final peace treaty was signed in 1998.
