Starship.
By TimeTraveller-1900.
It had been a long, hard fight for the last few years; ever since the aliens and the UFOs they used to travel through space to secretly arrive on Earth and began killing off humans to steal their organs to keep the human bodies they had been stealing for years alive while their intellect was put to the difficult task of keeping their race alive after a cosmic disaster, SHADO had known they couldn't lose their vigilance.
But the war had been flagging for a long time now; the aliens had been using ingenious methods of breaking through Earth's defences, but Commander Straker knew only too well they couldn't give up, but as their knowledge and understanding of the aliens had grown with each encounter, they had learnt even better ways of fighting them.
While many people if they had ever discovered what SHADO was for would-be stunned and even panicky at the prospect of an alien race preying on humans like they were cattle animals at a farm or an abattoir, the people who were behind SHADO knew there was no turning back.
They had to fight. And they had.
They had prevented many deaths and families from suffering; in the beginning, especially the first 10 years after the U.N. gave permission and funding for the creation of SHADO, they had needed to go to great lengths to ensure that every UFO destruction counted. But a cold war/stalemate existed between the aliens and SHADO; the aliens were more advanced than humanity, but they were being held off by the interceptors and SHADO was well practiced in the art of tracking down a UFO if it penetrated the cracks that existed around Earth's defences.
No more.
The moonbase personnel, a valuable part of SHADO's ability to monitor space and the satellites which provided them with their eyes and ears had successfully shot down a small number of UFOs; for a long time now, SHADO had been trying to acquire an alien spaceship in order to study their technology and to discover how to fight the aliens on a higher level, but they'd failed on so many occasions because the alien ships exploded in the atmosphere. In the end, it had been decided to capture a UFO in space, to see if it was possible and if the habit the UFOs had of being destroyed on Earth happened here.
Sadly the aliens refused to let themselves or their technology fall into the hands of humans. The interceptor pilots' crafts were equipped with smaller weapons, and while it was hoped they would just immobilise and damage the UFOs enough to stop them from moving and flying through space or through an atmosphere, the aliens destroyed their ships and took their own lives.
These constant failures were frustrating since General Henderson and Commander Straker wanted the stalemate which existed between SHADO and the aliens to break forever. Finally, it was decided to create a sting operation where SHADO's operations would be suspended. They set up a number of explosions on the moon, which would make it appear the moonbase had been destroyed, and SHADO was trying desperately to rebuild from what appeared to be a careless accident.
It worked.
The aliens, determined and desperate to acquire human body parts and tissues for transplantation and even a few human bodies if they were lucky, sent a number of UFOs to Earth. The aliens were not taking any chances; they had clearly monitored the nuclear explosion on the moon, and SHADO had not expected them to send three large spaceships that were larger and more powerful than their conventional UFOs with the aim of annihilating SHADO and making sure they were never a threat again. The UFOs made short work of the SIDs (Space Intruder Detectors) in orbit around Earth, and they split up, attacking the moon and SHADO control, while the rest of the ships went for deserted and remote towns and villages and settlements around the planet.
But the moonbase revealed its survival. They quickly galvanised and retaliated with new interceptor craft of a different make and model with more powerful weapons; the aliens had sent 7 of the larger UFOs to Earth to destroy SHADO, and 2 of them were shot down along with several of the smaller UFOs on the moon, where they were retrieved quickly with the help of SHADO personnel who had been advised to be on the moon surface in time before the attack; quickly they captured the alien ships and quickly overpowered the crews and isolated the ships.
On Earth the situation wasn't any better for the aliens; two Skydivers shot down three UFOs, destroying one of them but the other two crashed into the sea, where they were retrieved from the ocean.
The UFOs were defeated and the surviving ships retreated with heavy casualties, and their technology was now in human hands. SHADO quickly began stripping the ships of their technology; within 8 months, their technology was reverse-engineered and better space technology entered SHADO as well as NASA's hands and alien weapon technology allowed them to evenly match against the aliens.
Within a few years, SHADO began defeating the aliens more intensely; one of their later weapons was a weapon which involved spewing accelerated neutrons through space, which interfered with their technology and blew them up; this weapon would later form the basis of the Queller drive.
After a war spanning nearly 20 years, SHADO won the UFO war. But when the war ended, the U.N made the mistake of hiding the truth of where their advances were coming from.
Author's Note - Happy New Year! It's been implied in Big Finish's Space 1999, SHADO and Moonbase Alpha belong to the same universe; I decided to build on the idea.