Why The Godfather Part II works as a sequel

I'm actually more critical of The Godfather Part II then most people, it has many great scenes and performances and I love the premise of going back and forth between two time periods.  But I actually wasn't fond of it's execution of that premise, the two story-lines don't feel connected beyond that they're broadly about the same family.  The past sequences work better as a Prequel to the first movie then they do this movie.  And even if the film was just the present material it'd still feel more like episodes of a series packaged together then a complete film.

But I'm someone who likes to stay positive in my discussion of Entertainment Media I watch, so I instead want to focus on what I feel is an overlooked key to why The Godfather Part II works as a sequel so well, and in fact did more to justify the concept of cinematic sequels then any of it's predecessors.

First you need to understand that in the source material The Godfather had no sequel till 1984, and even that was more of a spin off, there was no proper sequel til 2004.  The past sequence of Part II is largely based on material from the original Novel left out of the original film, but the late 50s to early 60s portion was entirely new material, and Coppola actually had trouble convincing Mario Puzo to be okay with certain aspects of where the story went.

The film worked as a sequel by looking at where the first movie ended and deciding where to go next from there, something many sequels both before and since never really tried to do.

Because of this certain scenes in the later part of the first movie wind up seeming to a person who knows everything in 2019 watching them like sequel hooks when they could not possibly have been meant to be that at the time.  Like how Hagan is visibly unhappy with being increasingly left out of the loop on stuff.

But the big example is the Moe Green scene, after he leaves and Michael Says to Fredo

“Fredo, you’re my older brother, and I love you. But don’t ever take sides with anyone against the family again. Ever.”  

That scene is really eerie when you know where the sequel goes, and it's specifically that direction of the sequel Puzo was uncomfortable with so he certainly didn't write the scene intending it to foreshadow that.

And that aspect of how these two films connect to each other is something I feel gets largely overlooked.