You could use this event:
The Royal Air Force conducted the heaviest daylight bombing raid against Germany since the war began. The Germans could not offer as much opposition as they once did because many of their planes had been diverted to the Eastern Front.
The royal air force had used a wormhole, that day in order to attack; the wormhole is still open to this day troughthrough space and time and it is the only way the time machine can travel. Beware! In order to be consistent, your time machine should stop working if the wormhole somehow stops working, or being cut.