Species with identical needs cannot coexist — a small consistent advantage compounds until one is gone. Which means anything living side by side must differ, and finding how is informative.
Betraying your partner is the correct choice no matter what they do. So both of you betray, and both of you serve five years instead of one. No error was made — and that is exactly the problem.
Two petrol stations cut prices until neither can move without losing. Nobody wanted the result, everyone behaved sensibly, and it held anyway — because stable and good are different properties.
A rising power wants influence matching its strength; an established one sees each concession as erosion. Both are sincere, neither needs to want conflict, and the pressure builds anyway.