The Secretary of State insisted Cuba is a threat to the national security of the United States, although he did not provide evidence to support the claim.

Rubio made his remarks on the same day the Justice Department unsealed a murder indictment against former Cuban President Raúl Castro and the United States Southern Command sent the USS Nimitz carrier strike group to the Caribbean.

The indictment concerns the Cuban shoot-down of a Brothers to the Rescue (Hermanos al Rescate) aircraft in 1996. Brothers to the Rescue is headed by José Basulto and a group of Cuban exiles. Basulto admitted in testimony he is an anti-Castro CIA operative.

CBS News reported on September 25 that the US Army Reserve is examining whether it can provide the Southern Command with police forces, medical personnel, and other support units within the next four months, according to an internal Army message.

Though the message makes no reference to Cuba, the request is for continued military planning aimed at Cuba, according to multiple U.S. officials who spoke to CBS News under condition of anonymity because they were not authorized to speak publicly.

Prior to the Army message, the US conducted reconnaissance operations on the seafloor and maritime approaches surrounding the Cuban island. "Separately, U.S. airpower components were placed on heightened readiness and directed to pre-stage personnel for potential deployment. Those plans, however, did not come to fruition," according to CBS.

Cuba is "in a lot of trouble," Rubio said in May. He warned the US is prepared to invade Cuba if and when Trump gives the order. The US Navy build-up at the time was the largest deployment outside of West Asia.

These strategically placed assets set the table for military action, from a capture of Havana's leadership much like the seizure of former Venezuelan President Nicolas Maduro, to a series of precision strikes. And they open the possibility that the U.S. throws itself into the third international conflict of the Trump administration.

It is estimated the US would require more than 100,000 troops to conduct an invasion of Cuba and air and naval support would be essential for operational success. The Cuban Revolutionary Armed Forces maintain around 50,000 active soldiers and 39,000 reservists. It would not be difficult for the US military to deal with Cuba's military. However, urban combat and insurgency in rural areas would create significant challenges.

Following Trump's belligerent remarks earlier this year, Cuba's Civil Defense released a multi-page guide titled The Family Guide for Protection Against Military Aggression.