The Borg used a super epic time machine to make the universe go back 5 minutes, to before WWIII. Somehow, the girl retains her memories and pushes the child off the seesaw before he can detonate the thermonuclear device.
(See https://powdertoy.co.uk/Discussions/Thread/View.html?Thread=14221&PageNum=6#Message=208666)
But then it cause a paradox: the borg went back and stopped the nuclear detonation, preventing WWlll, but that means humanity never encounter the borg in the first place. This paradox rips the universe in half, with one half continuing with WWlll and the other continuing without WWlll.