A more comprehensive research program would look for historical cases of stable peace between countries that are not democracies, or between states only one of which is a democracy.
Stable peace among nations, by Arie Marcelo Kacowicz
Here, the author discuss generally about all countries, why he use “between”. He is ruling out the possibilities like world war two? Or he is emphasizing the abstract structure of war: two opponents, two polar opposites? According to N-gram, peace between countries is much more frequently used than peace among countries. What is your explanation?