I think that some of you are being a bit too harsh on Santino. Let's face it, the Don was slipping and he let Sollozo and company get to him. Something that he never would have let happen 5 - 10 years earlier. And Sonny was "thrown" into a
mess. The key was that Sollozo and company were able to take out Luca Brasi at the same time. The strongest force that the Corleones had. A good move that weakened Sonny and the Corleones quite a bit.

Now I'm not saying that Sonny was anything like his father, or his brother Michael in the cunning and leadership department. But he actually was a good wartime street warrior /
streetfighting / boss who's only real mistake was that he let his temper get the best of him on a personal matter which cost him his life.

Let's not forget ( and I'm taking the liberty of crossing over to the novel ) that years earlier he and Luca ran a street war and he did a good job of running and saving the family.

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.