The Andalite-Yeerk War (alternatively Yeerk-Andalite War) is a generic term applied to the war between the Yeerks and the Andalites. The war began in the Earth year 1966. The Andalites, under the command of Prince Seerow, had encountered the Yeerks on their homeworld. Prince Seerow felt sorry for them because they were an intelligent yet backwards race. He taught the Yeerks about science, technology, and even helped them to build portable Kandrona generators. Once they had this knowledge, the Yeerks betrayed Prince Seerow, attacking the Andalites on the Yeerk homeworld and escaping into space. The Yeerks intended to expand their empire by conquering and enslaving races across the galaxy. The Andalites, to undo the sin of Seerow's Kindness, went to war with the Yeerks, intent on stopping them.
The Yeerks traveled to the Hork-Bajir homeworld, where Prince Seerow had been exiled. In the Earth year 1968, they kidnapped many Hork-Bajir, using their arm and leg blades as weapons, even though the Hork-Bajirs' creators had intended them to be used to scrape bark off the trees. When the Andalites learned of the Yeerks' attack on the Hork-Bajir homeworld, Prince Alloran was dispatched to deal with the threat (it can be assumed the other Andalite warriors were preoccupied with the Yeerk infestation of the Mak, Garatrons, or some other species). Alloran realized that he could not stop the Yeerks from infesting the Hork-Bajir, despite his and the free Hork-Bajirs' greatest efforts. Therefore, he created a virus that wiped out all living Hork-Bajir. The only remaining living Hork-Bajir were Yeerk slaves.
After taking over the Hork-Bajir, the Yeerks traveled to the Taxxon homeworld in 1976. There they convinced the Taxxons that they would be better off as Controllers. However, many Taxxons resisted. At this time the soon-famous Prince Elfangor also journeyed to the Taxxon homeworld. However, he was distracted by the Time Matrix, and the Taxxons also fell prey to the Yeerks. Many Taxxons revolted later, in the Taxxon Rebellion.
Next, the Yeerk who would later become Visser One traveled to Earth in search of a Class 5 species. She found humans and documented them as such. Visser Three was then sent to Earth and began infiltrating it using "The Sharing", a front Visser One had set up. When the Andalite Dome Ship GalaxyTree was sent to investigate in 1996, Visser Three hid his Blade Ship behind the moon and attacked and destroyed the Dome Ship. Only two Andalites, Elfangor and Ax, survived. They both crash-landed on Earth. Elfangor gave five teenage humans the morphing ability. These five later joined up with Ax and started a guerilla movement against the Yeerks.
In late 1997, the Yeerks attacked Leera, in hopes of using the Leerans (who could read minds) as Controllers. However, due to the telepathic powers of the Leerans, they were unable to infiltrate Leera discreetly, and attempted to take over the Leerans by force. Visser One tried to use hammerhead sharks from Earth to use as shock troopers to storm Leera, but was thwarted by the Animorphs (the five teenagers given morphing powers by Elfangor). The Leerans were subsequently freed from Yeerk oppression.
In early 1999, Visser One was then sent to the Anati homeworld to defend it from attacking Andalite forces. After she failed a few months later, she was to be executed, but she escaped. She was later killed by her host's son. Visser Three was promoted to Visser One for stealing the morphing cube from the Animorphs. He made the Yeerk attack on Earth open in late 1999, and staged an all-out battle with the Animorphs. However, he was outsmarted, and was captured in the year 2000. With the arrival of the Andalite fleet and the death of 17,372 Yeerks at the hands of the Animorphs, the Yeerks' war effort was seriously undermined. (It can be assumed that they were already doing badly on other fronts, as well). The Yeerks surrendered to the Andalites, who allowed them to become nothlits.
Races Involved in the War
- Anati: The Anati are some of the few races able hold out against the Yeerks when they invaded their homeworld. They are allied with the Andalites, though the Andalites won't share their technologies with them.
- Arn: The Arn originally wanted to remain neutral in this war, but were forced to join the Andalites when Yeerks invaded their homeworld. Despite modifying themselves so they couldn't become Controllers, they were completely exterminated, and are thus no longer involved.
- Chee: The Chee are a small collection of non-violent androids. They oppose the (violent) Yeerks by acting as allies to the Animorphs.
- Desbadeen: Desbadeen are neutral in the galactic conflict.
- Dayang: The Dayang appear to trade with the Yeerks, thus they have some form of alliance with the Yeerks since it also appears they cannot be enslaved.
- Garatron: The Garatron were completely enslaved during the conflict.
- Gedd: The Gedd might have once been allies of the Yeerk, but like so many after them are now enslaved by the Yeerks.
- Great Ones: The Great Ones are neutral in the galactic conflict.
- Hawjabran: The Hawjabran are enemies of the Yeerks after they murdered 8,000 Hawjabrans when attempts to enslave them had failed. It's unknown if they are allies of the Andalites, though it seems likely they are.
- Helmacron: The Helmacron are a race bent ruling the universe and fight both the Yeerks and Andalites, although they are also currently in a civil war between revolting males and female oppressors.
- Human: Most Humans would fight against the Yeerks, but as a whole are unaware they are being invaded until the very end of the war.
- Kelbrid: The Kelbrid are neutral in the conflict, but will act aggressively toward the Andalites if they ever enter Kelbrid Space.
- Korla: The Korla's involvement in war is unknown.
- Leeran: The Leeran are allies of the Andalites and joined armed forces to rid the Leera of invading Yeerks. Despite being allies, the Andalites don't share their technologies with the Leerans, and always keep their distance from Leerans because the Leerans could read their minds.
- Little Ones: The Little Ones are neutral in the galactic conflict.
- Mak: The Mak were completely enslaved by the Yeerk Empire.
- Nahara: The Nahara were completely enslaved by the Yeerk Empire.
- Orff: The Orff are race resisting the Yeerks, though it's unknown if they have an alliance with the Andalites.
- Skrit Na: The Skrit Na were once targets of Yeerk enslavement, when the Yeerks attacked a Skrit Na Moon Colony. However, due their biology, attempts at enslavement have proved flawed. In some ways, the Skrit Na became responsible for the war making its way to Earth and getting humans involved when they abducted Loren and Hedrick Chapman in 1976.
- Ssstram: The Ssstram were completly enslaved by the Yeerk Empire.
- Taxxon: Most Taxxons are allies of the Yeerk Empire, and are voluntary slaves in exchange for exotic meats. However some rebels known as the Mountain Taxxons, led by The Living Hive and the Andalite Nothlit named Arbron (in Taxxon morph), are resisting the Yeerks, and joined the fight against them.
- Venber: The Venber, before the Andalite-Yeerk War, might have been allies of the Andalites until The Five wiped them out. The current Venbers are Human/Venber Hybrids, and are merely puppets of the Yeerks. The fate of these Venber are unknown, but they either died, or live in Antarctica.