I guess people would not use passive voice in english in this case, it sounds very strange, especially with "was begun". Passive voice is used when you don't want to name the person who did it, just the result, like "the apple was eaten", but you don't want to say who ate it. But here, you can express the same just by saying "the revolution begun", so you don't need passive voice.
Another thing is that the phrase "se inició" can be understood as the past of "iniciarse", which means to begin. In this case, even the spanish version is not meant in passive voice. Is that a correct interpretation?
Why it's 'was started'? Not just 'started' if the 'se' is used in the sentence?