So how did this happen? The Godfather is generally known and loved as one of the best movies ever made. Wanting to make a sequel, you take the leftovers you didn't film, some flashbacks, you create a new storyline on top of this, which is a VERY convoluted one and seemingly unrelated. And you assemble the same cast, well... except the most famous member of it, who refuses to return even for a cameo.
Doesn't really sound like a recipe for success, does it?
But The Godfather: Part 2 does the unthinkable and succeeds in every way possible. Knowledge of the first movie is an absolute necessity, you need to know, love, hate, dislike these characters before you start with part 2. But as the main storyline shows the moral downfall of Michael Corleone from bad to evil, the flashbacks show how the family got to be this way. The killer acting of everyone involved is a thing to behold. Add that brilliant ending, with its final shattering flashback, and this movie just delivers an emotional hit with the force of a jackhammer.
It even pulls off the remarkable feat of making the first film even better, by explaining things which didn't need to be explained but enrich it nevertheless. A stunning achievement.