Cuba threw a huge 80th birthday party for Fidel Castro, the country's leader for nearly five decades -- but the guest of honor did not appear, raising doubt as to whether he will ever rejoin public life.

The development furthers the succession of Castro's younger brother, Raul, who has been in charge since Castro fell ill four months ago.

Before the event, there were strong indications that Castro would appear. ABC News reported that a specially-constructed medical vehicle was brought to Cuba this week to enable Castro to be transported. A special abdominal brace was also created to support him while standing.

Observers now believe the decision not to appear was made at the last minute. Yesterday, a Cuban government official told ABC News that Castro's agenda was being set by his doctors.

ABC News consultant Marc Frank called today's no show a "defining moment," saying, "He would have appeared if he could, but he couldn't."

Questions About Cuba's Future

Castro underwent surgery on July 31 for an illness that remains a state secret in Cuba. Shortly thereafter, he gave a proclamation that implied he would attend this delayed birthday celebration, since his real birthday was Aug. 13. A man who prides himself on keeping his promises to the Cuban people, Castro's non-appearance has government searching for explanations and pondering the future.

Julia Sweig, director of the Latin America program at the Council of Foreign Relations in Washington, D.C., believes that while Cuban life has not been disrupted by Castro's illness, the succession has been smooth.

"Cubans have been planning it for years," Sweig said. "The people running the country today have been training with Fidel. Now they're managing the country without him."

Castro has reveled in the public spotlight most of his life, using state-run media to promote his Communist vision and to maintain his grasp on power. Havana billboards show his iconic image alongside slogans such as "Fidel es un pais," or, "Fidel is a country." In other words, he encompasses everything.

State run television is showing a tribute documentary, in which Castro appears in nearly every frame, marching, fighting, issuing fiery speeches and kissing Cuban children. Despite facing struggles in daily life to put food on their tables and ride the bus to work, the Cuban people appear concerned about his fate.

Cuban Vice President Carlos Lage Davila, known among Cubans as a man who speaks with authority, told a crowd of thousands last night at Karl Marx Theater that Castro was recovering and would "continue to lead" Cuba. While Frank says that Lage's word is trusted in Cuba, after today's surprise no-show, Castro's leadership has become an open and burning question in Cuba and beyond.

ABC News' Jim Avila contributed to this report.