An alternate dimension existed, completely different from the universe of Robotechnology, where various mechas known as Voltron existed to protect various different worlds. At least two of these are known to exist in relevant sources -- the Voltron Vehicle Force and the Voltron Lion Force, the protector of the planet Arus.
When the Omega Comet approached Arus, it made a connection between the two universes, leaving Arus unprotected for many years as the Lion Force was flung into Macross Island in the Robotech Universe. Years later, an attempt to bring the Force back by Lotor and the Drule witch Haggar instead forced the SDF-1 into their universe.
The Lion Forces fought off the enemy Zentraedi, and the SDF-1 fought with the Drule Empire, before they both eventually came into contact with the Comet again, resetting their universes and undoing the timeline. (Robotech/Voltron 1, 2, 3, 4, 5)