"If the Borg can travel back in time to change history whenever they want, why didn't they do it earlier?"
Just have them go back in time to change when they started going back in time...

Maybe this should have gone in the Post-of-Many-Names...
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The Unpronounceable Krsqk
"Well, sir, at the moment my left processor doesn't know what my right is doing." -
Freefall