Sol knew that Sonny had lots of people hunting for him. He hoped that, by meeting with Michael, he'd at least put some kind of phony proposal on the table that Sonny would consider--maybe delaying retribution for long enough for Sol to get Vito.
Sol and Barzini knew Sonny'd get mad after the first hit on Christmas eve. But if Sollozzo killed Vito in a second attempt after the "truce", no-one would be able to stop Sonny. Not Michael, not Tom, no political considerations.
Sollozzo and Barzini must have known that. Which means, they had been prepared for a war already.