The ambush did not emerge from nowhere. Since the mid-1960s, paramilitary groups tied to the Puerto Rican independence movement had been carrying out attacks on U.S. interests, both on the island and on the mainland. Most targeted property and infrastructure. By 1976, after an earlier generation of militant groups had been dismantled, new organizations emerged: the Boricua Popular Army (EPB, known as Los Macheteros), the Armed Forces of Popular Resistance (FARP), and the Organization of Volunteers for the Puerto Rican Revolution (OVRP). In September 1979, five such groups formed a joint National Revolutionary Command. Their first coordinated action, on October 19, struck six targets in Puerto Rico and Chicago simultaneously.

Three weeks before the Sabana Seca ambush, on November 11, 1979, Puerto Rican independence activist Angel Rodriguez Cristobal was found dead in his prison cell in Florida. He had been serving a sentence for protesting the U.S. Navy's use of Vieques island for bombing practice - a cause that would eventually draw broad public support and lead to the Navy's withdrawal in 2003. Authorities ruled his death a suicide. Many in the independence movement believed it was a political killing and vowed retaliation. The communique issued after the Sabana Seca attack cited Rodriguez Cristobal's death as a direct motivation, alongside the killings of two pro-independence activists at Cerro Maravilla in 1978 - an event that later investigations revealed involved police entrapment and extrajudicial execution.

John Ball and Emil White were communications technicians stationed in Puerto Rico - military personnel doing technical work at a listening post, not combat soldiers. They were unarmed on that bus, as were all 18 of their colleagues. The violence of the ambush - 47 rounds fired in half a minute - was designed to make a political statement, but the cost was borne by individuals. Ball and White had families, careers, lives beyond whatever symbolic value their attackers assigned to them. The National Security Agency later memorialized both men for their cryptologic service. Their deaths illustrate how political violence, whatever its stated justifications, falls hardest on the people who happen to be in its path.

The political response was immediate and bipartisan in its condemnation. President Jimmy Carter called the ambush "a despicable act of murder." Puerto Rico Governor Carlos Romero Barcelo pledged to find the perpetrators. Even the Puerto Rican Independence Party, which supported independence through democratic means, condemned the attack, warning it would "scare some people" away from the cause. Security at military installations across the island was tightened, and Navy personnel were advised not to wear uniforms off base. The aftermath took darker turns: in 1980, two suspected participants were assassinated, allegedly by right-wing paramilitaries. It was not until 2014 that Juan Galloza Acevedo was sentenced to five years in prison for his role in the attack - 35 years after that morning on Route 867.