While the Mad Titan Thanos has been defeated multiple times by the various heroes of Earth, there's a reason he has never simply destroyed the planet which gives him so much grief. Given Thanos’ obsession with Death, it's in his nature to wipe out anything that opposes him, but he's gone as far as protecting Earth from this fate before, and for a very specific reason

Thanos is one of the most powerful villains in the Marvel Universe, an interplanetary warlord with massive armies at his disposal. He has immense physical abilities and can also shoot cosmic energy beams from his body. Thanos is also incredibly smart and knowledgeable in science and technology far surpassing that of Earth. All of this is without his habit of claiming powerful cosmic artifacts. While he is no longer in possession of the Infinity Gauntlet, his most iconic weapon, he has accessed similar weapons like the Cosmic Cube and Heart of the Universe before. Nevertheless, Thanos has never actually destroyed the Earth despite having the ability to do so. The reason for this gives a lot of insight into his character and motivations.

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In Guardians of the Galaxy #19, written by Brian Michael Bendis with art by Valerio Schiti, Richard Isanove, and a vast team of guest artists, Annihilus, the Brood Queen, and a Badoon representative try to get Thanos to join them in an alliance to destroy the Earth. They reason that if the Earth and its various heroes are no longer in the picture, the galaxy will be easy to conquer. Thanos agrees to join their forces, but on the condition that they don’t destroy the Earth, but instead conquer it. While the others tell him that this sort of thinking is exactly what has led to their past defeats at the hands of Earth’s protectors, Thanos is adamant. He says that the planet needs to be put under his submission and that once it is, he will use it as the center of his empire.

Thanos guardians of the galaxy annihilus brood

Thanos’s unflinching stance on this point reveals a lot about his nature and driving motivations. Given his genius-level intellect, he probably realizes that destroying the Earth would be much more convenient for him in the long run and enable him to accomplish his plans. But despite having had the power to achieve this goal on many occasions, his pride won’t let him take this logical path. Thanos must see the heroes and world that has humiliated him so many times brought under his control and bent to his will, not just destroyed.

Thanos' pride reveals a major flaw that stands in the way of him accomplishing his goals, and makes him vulnerable to future defeat. With his various cosmic weapons, Thanos could have destroyed the Earth on several occasions before anyone even knew he was a threat. However, because of his desire to conquer those who have embarrassed him in the past, Thanos refuses to take this path, and will even dissuade others (like Annihilus) who intend to eradicate his prize. In a twisted way, Thanos is a major ally to the Avengers in keeping the Earth safe from galactic threats, albeit only so he can take the planet for himself.

At the end of Guardians of the Galaxy #19, after Thanos and his invasion of Earth are defeated yet again, Annihilus and the Badoon representative both agree that they’re better off just leaving the planet alone. Based on his overwhelming desire to subjugate Earth and its heroes revealed in the same issue, Thanos will likely never be able to do this. So, Thanos’ reason for not eliminating the Earth not only greatly characterizes the Mad Titan, it also sets him apart from Marvel’s other extraterrestrial villains, making him a constant antagonist to a planet he refuses to simply destroy.

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