The conflict in Serbia between Austria-Hungary and Russia help ignite World War I because Europe split into Allied and Central powers as countries honored alliances to fight together if attacked. The balance was built in a way that just a sparkle would have be enough to cause a big flame. The conglit i Serbia was that sparkle.
It depends on what you're referencing it to. the domino theory, in one historical aspect, was referenced during the Vietnam war. It was meant to say that if the US lost Vietnam to the communists, then countries like Cambodia, Burma, and eventually India would fall to the communists.
