This is like how John Connor sent Kyle Reese back to 1984 to protect his mother Sarah Connor, so that she could give birth to the leader of the human resistance, John Connor. But Reese goes back to 1984 and falls in love with Sarah Connor and gets her pregnant with the baby that will grow up to be John Connor. So if John Connor didn't send Reese back in time to protect Sarah, John would have never been born in the first place and wouldn't have existed to lead the humans and send Reese back in time to protect his mother.
And this, the theory of parallel universes is told. At some point in the nanosecond this decision continues to be made, an atom will shift this disrupting the reproduction of this exact scenario. It may play out billions of times a second because time doesn't stop therefor eventually John Conner is never born, we cease to exist, yet we continue to go on in the smallest fraction of time.