The archrivals the Masters of Evil, in a storyline which featured supervillains taking over Avengers Mansion, putting some of the team’s most powerful heroes out of action.

The story of how the Masters of Evil beat the Avengers was told in Avengers #273-277 – written by Roger Stern, with art by John Buscema – in the “Under Siege” arc, which is widely considered among the greatest Avengers storylines of all time.

The story starts out with the team having to deal with the unpopularity of Namor, who eventually decides to leave the team following a public outcry and mounting responsibilities in Atlantis. During a protest over Namor’s hip, one woman trying to stir up trouble is revealed to be the villain Moonstone.

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Avengers #273-277 – Written By Roger Stern; Art By John Buscema; Released In 1986

In the story, Moonstone flies way after a brief skirmish with the Avengers, but it i revealed that she’s part of a larger scheme: the Masters of Evil are being re-formed under the leadership of Baron Helmut Zemo, the son of the original who founded the initial version of the super-villain team. After several more small skirmishes with seemingly random villains, Zemo gathers his forces together to take over Avengers Mansion. The villains capture the Avengers’ faithful butler Jarvis, and proceed to ambush each member as they unknowingly walk into Zemo’s trap.

The Avengers line-up during “Under Siege” consisted of the Wasp, Captain America, Hercules, the Black Knight and Captain Marvel (Monica Rambeau)

Things continue to get pretty dire for the Avengers, as Zemo enacts a dastardly plan to deal with the team’s powerhouses. Monica Rambeau is subdued by Blackout, who traps her in a prison of Darkforce energy, before encasing the entire Mansion in the same energy. And after paying a woman to spend the night with and drug Avengers' hero Hercules, Zemo unleashes his own powerhouses Goliath, Hyde and Tiger Shark to beat the Olympian into a coma. Subsequent attempts by the Avengers to retake their home result in Captain America being held prisoner, leaving then-team leader Wasp to deal with the threat.

With a little help from reinforcements Scott Lang, Thor and Doctor Druid, the Wasp is able to break through the Darkforce energy and free her captive team-mates, leading to an epic confrontation between Zemo and Captain America. While the Avengers ultimately triumphed at the end of the story, the Masters of Evil showed how vulnerable they truly were. It’s a story lacking the galactic stakes of stories like The Infinity Gauntlet, but the Avengers were forever haunted by the time Baron Zemo and the Masters of Evil almost destroyed them outright.

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The trauma of “Under Siege” lingered for years thereafter, but Baron Zemo wasn’t done yet. Although his takeover of Avengers Mansion was ultimately unsuccessful, Zemo later form the Masters of Evil once more in a way that would change the Marvel Universe forever. It was following the battle against Onslaught, when the Avengers and the Fantastic Four were presumed to have been killed. In actuality, they had been sent to a pocket universe, but that left the mainline Marvel universe with a severe lack of superheroes, resulting in a new team known as the Thunderbolts to appear in the other heroes’ absence.

Zemo saw the disappearance of the Avengers and the Fantastic Four as an opportunity to gain the public’s trust as a brand-new team of superheroes.

As revealed at the end of Thunderbolts #1, the new team was actually Zemo and the Masters of Evil in disguise. Zemo saw the disappearance of the Avengers and the Fantastic Four as an opportunity to gain the public’s trust as a brand-new team of superheroes, allowing him access to the Baxter Building and the entire SHIELD database. The Avengers and the Fantastic Four eventually found their way back to their home universe, and Zemo’s scheme was revealed to the world at large after the Thunderbolts’ true identities were revealed.

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What’s most interesting is that this group was made up of Zemo, Goliath, the Fixer, Moonstone, Screaming Mimi and the Beetle; many of the same villains who were involved in the “Under Siege” story-line. Many of these villains found that they actually enjoyed being heroes during the “Thunderbolts” ruse, and decided to continue being heroes under the leadership of former Avenger Hawkeye. The name has also gone on to be used by several different groups of villains and heroes since, but most incarnations have had at least one of the original, reformed Masters of Evil on their roster.

In a roundabout way, the Avengers’s greatest defeat ultimately led to the creation of the Thunderbolts. While the team has never had the overall name recognition of the Avengers or the X-Men, they have been an important piece of Marvel Comics history for several decades now. With the team set to make their big screen debut, the Thunderbolts are about to get more widespread exposure than ever before, so it’s important to go back to the source and see their beginnings during one of the most humiliating defeats the Avengers have ever faced.

Avengers #273-277 are available now from Marvel Comics.