The Avengers: United They Stand (also known simply as The Avengers), was an animated series based on the Marvel Comics superhero team the Avengers. 13 episodes in length, it originally aired from October 30, 1999 to February 26, 2000, and was produced by Avi Arad and distributed by 20th Century Fox Television.
The series featured a team broadly based on the roster for the 1984 Avengers spin-off series The West Coast Avengers, comprised of the Wasp, Wonder Man, Tigra, Hawkeye, and Scarlet Witch (Hawkeye and Scarlet Witch were also both in the Iron Man animated series as members of Force Works), led by Ant-Man (aka Giant-Man), with the Falcon and the Vision joining in the opening two-parter. For undetermined reasons (perhaps due to their rights being tied up in planned movie projects), the Avengers' "Big Three" were not regular fixtures in the series - Captain America and Iron Man made only guest appearances in one episode each, while Thor did not appear outside of the opening titles.
The series featured many of the Avengers' major comic book foes, including Ultron, Kang the Conqueror, Egghead, the Masters of Evil, the Grim Reaper and the Zodiac, as well as associated characters such as the Swordsman, the Circus of Crime, Namor, Attuma, Agatha Harkness and the Salem's Seven. The show made several fan-friendly references to aspects of the characters' comic book history that were otherwise not expanded upon for the uninitiated, such as the Falcon and Captain America's partnership, Hawkeye's partial deafness or Namor's half-breed nature.
Beyond this, however, the series bore little in the way of similarity to the comics, mainly due to its wholesale redesign of the cast, characterized by asymmetrical costume design and the most (in)famous element of the series - Ant-Man, Wasp, Hawkeye and Falcon all wore suits of battle armor, which they donned in sentai-inspired "power-up" sequences.
The entire cast from X-Men: The Animated Series were going to appear in a planned, but unmade two part episode during Season 2. The series was, however, cancelled before Season 2 was made. The X-Men were to have be voiced by their respective voice actors/actresses. Some of the voice cast for Avengers started from X-Men. Lenore Zann and Tony Daniels, voices of Rogue and Gambit, for instance voiced Tigra and Hawkeye.
The first two issues are set before the series premiere, Avengers Assemble, Part 1 & 2. #1 has the Avengers injured by an early version of Ultron. This issue was used to explain the armor worn by Hawkeye, Wasp and the Falcon in the series, which uses Pym Particles to reduce the pain caused by injury. It is mentioned by Hawkeye that Hank built his own armor after his legs were broken by Dragon Man, and felt no pain in his legs until he removed the armor. #3 takes place after the premiere as the Vision is being interviewed about his membership. He is also briefed on how Wonder Man and Hawkeye joined the team.
The Black Panther would appear in #1 and 6-7 of the series. In #1 he is among the Avengers who are injured by Ultron. Because of that, he refuses to rejoin the team until Hank steps down from being leader. Captain America appears in #6-7 as well. Quicksilver is mentioned twice in the series. In issue 1, Wanda is shown writing a letter to him. Issue 3 shows him in the flashback alongside Captain America, Hawkeye and Scarlet Witch.