Averaging 50 feet in height, the Zentraedi were created for one purpose; war. The Robotech Masters, desperate to defend themselves from the Invid, created the slave race in an attempt to survive.
After the betrayal of the Invid by Zor, and his sending the SDF-1 to Earth, the Zentraedi were dispatched to find the ship. The fleet led by Commander Breetai entered Earth's solar system and were detected by the SDF-1, and the ship autonomously fired its main gun, destroying several Zentraedi dreadnoughts. They immediately retaliated, and the First Robotech War began.
Over the course of their chase of the SDF-1 through the system after the ship folded to Pluto, the Zentraedi became exposed to ideas and emotions they had never imagined existed. Designed for war, they had no knowledge of concepts like love, music, or family. The more they were in contact with humanity, the more curious they grew about such things, dispatching spies to try and discern the purpose of behavior they were unable to explain.
The capture of Rick Hunter and Lisa Hayes gave them their first glimpse of love in action, as they forced the two humans to kiss in demonstration. Needless to say, they were horrified by the display. Alternatively, Hunter and Hayes used their incarceration as an opportunity to study the Zentraedi as well, and were surprised by the horrible condition of the ships. The Zentraedi dreadnoughts were literally falling to pieces in certain areas, the aliens having no idea how or even that they should make repairs. The intelligence Hayes gathered was crucial in opening peaceful negotiations with the Zentraedi.
Additionally it was found that Zentraedi and humans were virtually identical on a genetic level. This led to speculation regarding the creation and evolution of humanity, but on a more practical level, it allowed humans to better understand their enemy. The two species were so similar that cross-species procreation was possible, leading to the birth of many "half breed" children, including Dana Sterling, hero of the Second Robotech War.
The First Robotech War ended with both peace and battle. Supreme Commander Breetai and his advisor Exedor were more than willing to hold peace talks with the humans, whom they called "Micronians", but others were not so compliant. The egomaniacal Khyron broke ranks with Breetai, incensed by what he saw as betrayal by Breetai and even some of his own troops by wanting to end the war, contacted Supreme Commander Dolza of the Zentraedi Armada. Dolza gave Khyron the troops and weaponry he requested, including the genocidal Rain of Death.
Eventually, the renegade Zentraedi and the human forces managed to defeat Dolza and Khyron, ending the War and allowing reconstruction of the devastated Earth to begin. The Zentraedi revealed their creators, the Robotech Masters, would not be long in coming to Earth to retrieve the protoculture matrix on board the SDF-1. However, their utter lack of any technological training made them useless in revealing further secrets about the matrix.
The attempt to integrate Zentraedi into Earth culture was attempted with highly mixed results. Many made a successful transition, either micronized or retaining their original size. Just as many, however, found the peace time impossible to deal with. Zentraedi related violence became widespread. The decision was made, the Zentraedi who wished to leave would be allowed to do so, to make their own way in the galaxy. Nothing was heard from those Zentraedi after they left.