Besides all good references you see here, try to understand human beings as selfish creatures. Cooperation comes into being when you need to play with the same person/team many more times, that is, iterative games. If other party, say, X just works with you for one time, X can use you and dump you, while being nice at the same time. However, if you and X are forced to work together many more times: X and you have to cooperate.

It is easier to backstab than being openly inimical. Being openly inimical has both emotional and political costs; backstabbing is easy.