We can really ask the question of why didn't Vito order Sonny/Tom to give in and make the same plans with them as he later did with Michael. The story wouldn't have been as interesting of course.
I think that had Sonny still had Luca Brasi at his side, and a wartime consigliere like Genco, things would have been much different. I think that he just may have successfully wiped out most of his enemies and may have been able to sustain the family during the war until the Don was healthy enough to get back in the chair.
I think he was depending on Sonny to mount a holding action until he got better and could take command.
And I agree with Turnbull in that Vito was depending on Sonny to hold things together until he was healthy enough to take back control.
In the novel Sonny does not consider the murders of all the other Family heads until the Corleone Family position becomes dangerously precarious. The Family is somewhat overmatched. The Don's political network is neutralized; Tom is not quite a wartime consigliere; Tessio has mellowed and Clemenza lacks "youthful energy". Sonny had wanted to fight a holding action until his father could rejoin the fray but events on the ground made that impossible. Sonny was nearing a point where he had to either "go long or go home".
We agree on that.
Yes, Vito was depending on his eldest son to hold things together.
And Sonny was not as smart or as cunning as Vito or Michael.
All I'm saying is that Sonny's hand was forced in a way that Michael's wasn't. So his aggressive defense of Family power was the best move that could be made at that time.
If Vito had given Sonny permission to assent to the drug deal (for strategic purposes) and Sonny refused because he wanted immediate revenge that would be a different story.
