I remember it being explained to me as if Professor X, Magneto, The Fantastic 4, Wolverine, multiple man and more were on the same team. I was all 'gimme and gimme NOW!'.
(pre crisis legion stops in the 80's but AFTER the crisis. Volume 3 issue 30 is one of the most convincing break arguments. Although stuff like the the Magic Wars may have happened to this Legion but not in the way we saw it. I would also advise against starting with the preboot for new readers, there is too much of it and not all of it is available)