In terms of military air traffic, there are only two airbases that would feel the effects of the S-400: Naval Air Station Key West and Homestead Air Reserve Base.
The S-400 Triumf (NATO reporting name SA-21 Growler) is a mobile, surface-to-air missile system (SAM) designed by Russia. It is capable of engaging aircraft, UAVs, cruise missiles, and has a terminal ballistic missile defense capability. It represents the fourth generation of long-range Russian SAMs, and the successor to the S-200 and S-300.
According to Missile Threat CSIS Missile Project, the longest reach for the S-400 is 400 km with speeds of around Mach 14, giving the jet fighters little time to respond.
Now, given this defense system unique capabilities would a theoretical S-400 deployment to Cuba pose a credible threat to the U.S. air assets based on the country’s east coast?
As Larry Sprott, an aviation expert, explains on Quora “One of the things it can’t do is ground the US Air Force.
“The main threat of the S-400 would be to civilian air traffic in and around Miami. In terms of military air traffic, there are only two airbases that would feel the effects of the S-400: Naval Air Station Key West and Homestead Air Reserve Base. Naval Air Station Key West is mainly a training facility and one of the facilities that conduct counternarcotics. Homestead Air Reserve Base is a base that has a small contingent of Florida ANG F-15C Eagles and between 12–24 F-16C/D Fighting Falcons, so around 24–48 4th gen fighter jets. 4th gen fighter jets are vulnerable to systems like the S-400, but all is not lost.

“Out of harm’s way is MacDill Air Force Base which is home to 2 aerial refueling planes, which means that it can assist in bringing in more forces. Planes that would be integral in dealing with the S-400 would be the F-22 Raptor, the F-35 Lightning II, and B-2 Spirit. All of those planes are stealth planes that have been shown to slip past the S-400. In addition to this, the AGM-158 JASSM could be used as a standoff missile to take down the S-400 since it is a low observable cruise missile, but it also allows for 4th gen planes to be used in fighting the S-400. In addition to this, there are multiple naval bases in the area, so this brings the option of Naval Strike Missiles being used to destroy the S-400.
“In short, the S-400 would be a nuisance for a very short amount of time, assuming the US would let it be sold to Cuba in the first place, assuming Cuba could afford it (it would literally take around 43% of their defense budget to buy one). At most, it would mainly make the US very upset and would lead to really tense diplomatic talks. Russia isn’t stupid so they’d never try that unless they were planning to fight the US, which is another story in itself.”
Sprott concludes:
“The reason why one could think that the US would be so threatened by placing an S-400 there is because of the Cuban Missile Crisis. The difference is in the fact that the missiles in the Cuban Missile Crisis were the R-12 Dvina [NATO reporting name SS-4 Sandal] theatre ballistic missiles and the R-14 Chusovaya [NATO reporting name SS-5 Skean] intermediate-range ballistic missiles. These were nuclear missiles that could attack most of the mainland US. The S-400 has neither the reach nor the firepower to do this.”

Photo credit: Master Sgt. Jeremy T. Lock / U.S. Air Force, Ministry of Defence of the Russian Federation and Vitaly V. Kuzmin via Wikipedia