Pete, you know you surprise me -- if anything in this life is certain -- if history has taught us anything -- it's that you don't ever take sides with anyone against the Family Ever.

We see enough of Ola-Roth side of the chess game to deduce they are the main adversaries and their linking to the shooters

We already know by the time of the phone call,
1. Vito never trusted Roth or his Sicilian messenger boy, Ola
2. It was Roth that tried to have Michael killed

Then the phone call,
1. He'd already helped them out and they need some more
2. you got me in deep enough already How so? Why because he helped set Michael up to be murdered?
3. Ola's assurance Just go along, everything will be alright this time, not like before when the shooters messed up

We see enough of the Mafia to speak with this sort of certainty that anyone born and bred, would know they can't set up shop, outside their Family, on their own

How do you solve a problem like Fredo? Roth obliged by attempting to kill Michael
He knew he'd be stuck with running Mickey Mouse nightclubs until Michael dead and Fredo the new Don