By the way, what is the meaning of these sentences?
I'm puzzled by the two possible meanings: 1. We have looked at the event within some context unrelated to world history, but now we need to take into account the world history.
2. We have looked at the event within a context of world history, but now we need a broader context.
Just like the Webster's did, I would also use the definite article here, i.e. the broader context of the world history. Since it refers to the whole world history, which clearly sets the context for events
I would use indefinite article if I was just saying: ...must be looked at within a broader context.