Star Trek: 10 Things You Didn’t Know About The Guardian Of Forever
7. It Has The Power To Cross Universes As Well As Time
This is the crucial piece of information that Terra Firma revealed to the audience: the Guardian, or Carl, has the power to send people through time and across universes.
The Temporal Wars of the late 26th and 27th centuries threatened to tear the Milky Way galaxy apart, rippling back through time so that even the crew of the first Enterprise were affected. In those wars, factions discovered the power of the Guardian, and attempted to use it to gain the upper hand in the fight.
Perhaps the Guardian was always able to cross universes, or perhaps this was something that it developed in the almost millennium between Kirk and Spock encountered the being, and Georgiou nipped the Mirror Universe and back.
Either way, the power at the hands of the Guardian is potentially devastating, and most certainly not something one would want falling into the hands of the Terran Empire.