"The Vatican has not become a mediator in either Ukrainian or Syrian disputes." The pope will not stick his neck out here.
The rapprochement between Cuba and the US was initiated as soon as Obama became president. He began by easing some trade and travel restrictions in 2009. Negotiations between the two countries ensued.  Even the pope said, “[Rapprochement] was the goodwill of the two countries, and the merit is theirs, the merit is theirs for doing this. We did hardly anything.”
Pope Francis refused multiple requests for his participation or presence during the Colombian peace process. In October 2016, the pope said: “I have said that when the peace process [in Colombia]… if it succeeds, I would like to go, when everything is ‘airtight.’