Thanos has come close to controlling all of reality and defeating the Avengers permanently on multiple occasions, but his biggest weakness has prevented him from doing so. In a confrontation with Adam Warlock, Marvel Comics revealed that the Mad Titan was ultimately losing his greatest battles despite coming ever so close to winning them because of his ego and lack of worthiness. Thanos is his own biggest weakness.

Few villains in the Marvel Universe have had as close to complete and utter control of reality as Thanos has. Most famously, he obtained all of the Infinity Stones in the Infinity Gauntlet Saga, which he used to shape the cosmos in his own image and snap half of all living things from existence. Despite putting himself in the position to be a reality-altering god, Thanos has badly fumbled his unparalleled control each time he's nearly mastered it - often allowing his own hubris and desire of Death's love and affection to get in his own way.

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In Infinity Gauntlet #5 by Jim Starlin, George Perez, and Josef Rubenstein, Adam Warlock asks for Thanos' help in taking down his daughter Nebula, who just acquired the Infinity Gauntlet. When Thanos balks at the idea, Warlock reveals he knows Thanos better than anyone thanks to his time spent with the Soul Gem. Warlock tells Thanos that he sees a man who gains the ultimate power of the universe and loses it as soon as he acquires it - because "deep down in his soul he knows he is not worthy of acquiring it."

Thanos Marvel Comics Avengers

Warlock verbally undresses Thanos, telling him that he's acquired incredible, universe-shaping powers three times, and every time, he's been defeated by himself. He tells the Mad Titan he allowed Nebula to take the Infinity Gauntlet just as he previously allowed Captain Marvel to destroy the Cosmic Cube. Thanos struggles with his response, uttering out that "even gods err.." before grabbing his head in disgust and agreeing to help Warlock stop Nebula.

The scene shows why the Avengers almost always defeat Thanos, despite the odds stacked wildly against them: He's his own biggest weakness. Thanos has had total control of the Marvel Universe and lets his desires and love for Death torpedo his ultimate goal - even when it seemed unlikely he would lose. As long as Thanos continues to self-sabotage and give his opponents an opening to take him down when all hope is lost, the Avengers will always be victorious against him - no matter how impossible things seem. Thanos is his own worst enemy when it comes to accidentally creating in own defeats.

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