When ponies started mass producing warp ships, Princess Luna was authorized by her sister to start a space exploration and military organization. This was the Equestrian Airospace Cooperation or EAC. After begging Luna for a position, Princess Cadence was given the position of head of Mars base operations.
Equestria suddenly underwent a unexpected power shortage. Desperate to find a new energy source, Cadence looked to old culture for help. She develops a plan to open a portal to Hell to harvest argent energy. This portal would be opened by a subspace energy field. This would be known as the Lazurus project.
The Lazurus project may have been the best thing that ever happened to ponies. Their knowledge grew exponentially. Studying demons lead to breakthroughs in armor and weapon technology. They were able to get nutronium from tritanium using the same subspace technology they used to open the portal.
But some ponies fell under the demon's influence. A satanic cult had emerged. This went under the radar for a while, until the leader of the cult, Aurora Foul, made her move and started the demonic invasion. Most of the Mars base occupants were killed in the initial invasion.
EAC marine, Buckshot Blast, and a group of survivors tried to make their way to the Argent Tower before Aurora to keep her form opening the final gate to Hell.