Iron Man has always been an irreplaceable member of the Avengers. He's funded the team with his billions, he staffed the team's support with the loyal Edwin Jarvis (a flesh-and-blood butler rather than the A.I. assistant from the films), he fought back-to-back with other Avengers against impossible odds, and even served as chairman a time or two. Along with all that, he's helped the team in a way that he never expected or intended: by unwittingly helping to recruit future members of the Avengers — even future leaders of the Avengers — by fighting them.
Clint Barton, a.k.a. Hawkeye, first found his way into the Marvel Universe in 1964's Tales of Suspense #57. He appeared not as a guest star, but as a villain. Enamored by the seductive Black Widow — who herself was not exactly on the side of the angels back then — Hawkeye was willing to do anything the Soviet spy asked, including fighting Iron Man to help her steal Stark's technology. Eventually, Black Widow abandoned Hawkeye and the archer realized he didn't want to be a criminal anymore.
A year later, in Avengers #16, Hawkeye became part of "Cap's Kooky Quartet" – along with Captain America and two other former villains, Quicksilver and the Scarlet Witch — but it would be a while before the archer and Iron Man would trust each other. Often arrogant and rebellious, Hawkeye left the team almost ten years later and briefly joined the ranks of the Defenders right before one of Marvel's first big events — The Avengers/Defenders War — broke out. When Avengers and Defenders paired off against each other, it was Hawkeye and Iron Man who fought in 1973's Defenders #9. Throughout the fight it was clear that though almost a decade had passed in the real world, plenty of animosity still existed between Barton and Stark.
Fences were eventually mended enough that not only did Barton and Stark trust each other as teammates, but Iron Man would agree to work under Hawkeye's leadership in the first incarnation of the West Coast Avengers.
