Very Astute Tony B.

Tom knew the Tattaglias were backing Sollozzo, but he did not figure that Barzini was also in on it. This was also a blunder.
I think the way Genco would have handled it to avoid a war would have been to approach the consiglieres of the other families and tell them unequivocable the Corleones would not risk their political and judical protection for this drug pusher who was running around.

After that I am sure workd could have gone to the Police that there was this new mafioso in town trying to set up a drug operation. Since he already had a prison record in Italy, he would be lucky to be deported, and likely to be wearing cement boots in the east river.