In his first official government appearance since illness forced him to relinquish power four years ago, Castro asked world leaders to urge President Barack Obama to avert a nuclear war with Iran.
"They would be ordering the instant death of hundreds of millions of people," Castro said.
With his brother Raul - the current Cuban president - taking notes, the surprisingly vigorous 83-year-old seemed to hint at a possible change in U.S.-Cuba relations.
"They are creating at this moment the conditions for a situation not even dreamed of a short time ago," he said
Last month, Cuban officials agreed to release 52 political prisoners by mid-November. So far, 21 have been set free.
And in Havana there are rising hopes for a possible prisoner exchange that could include five Cuban agents convicted of spying in the United States.
Still, experts do not believe a major shift is in the works.
"I don't think there's going to be any kind of sea change in the relationship," said Peter Deshazo, director of the Americas program a the Center for Strategic and International Studies, but "it could very slowly evolve in more positive direction."
And the return to the spotlight of "el Comandante" may mean an unwelcome return to the status quo - and a continued vigil for Cuban-Americans waiting for meaningful diplomatic moves.