I think Sky Fortress is a group of satellites, probably including Iridium for tracking targets and communicating with other satellites and with SpaceX as the developer. The Department of Defense already pretty much owns Iridium and the military uses SpaceX often.

That new book Fire and Fury also mentions a meeting between Elon Musk and Donald Trump. Mentions them discussing the mission to Mars. Is it possible that they will get this contract as a reward for launching Iridium/Space Fortress?

This theory also provides a link between the tweet about Fire and Fury and Ronald Reagan, who launched the strategic defense initiative, described below:

"The Strategic Defense Initiative (SDI), also known as Star Wars, was a program first initiated on March 23, 1983 under President Ronald Reagan. The intent of this program was to develop a sophisticated anti-ballistic missile system in order to prevent missile attacks from other countries…The nickname “Star Wars” may have been attached to the program for some of its abstract and farfetched ideas, many of which included lasers…weapons required included space- and ground-based nuclear X-ray lasers, subatomic particle beams, and computer-guided projectiles fired by electromagnetic rail guns—all under the central control of a supercomputer system.” By using these systems, the United States planned to intercept intercontinental ballistic missiles while they still flew high above the Earth, minimizing their effects. However, there was a large power requirement for these types of weapons…At the end of the Strategic Defense Initiative, thirty billion dollars had been invested in the program and no laser and mirror system was ever used, not on land, not in space."

Wow so nothing came of this program? Not quite.

The US successfully tested the new technology in 2012 against targets rigged up with computer systems and “representative weapons of mass destruction production equipment” in North Korea and Iran, according to a declassified Air Force report. CHAMP - which stands for Counter-Electronics High Power Microwave Advanced Missile Project - works by emitting sharp pulses of microwave energy which fry electronic circuits. There were news stories all over the internet a month ago about the military, suggesting it could be used against NK and saying it could be mobilized. Would have to be shot from a B52 Stratofortress though, which could be another possible meaning for Sky Fortress and explain the DEFCON 1.