The Attack on Mars was a surprise attack which occurred on April 5, First Contact Day, in 2385. It was masterminded by Commodore Oh, a Zhat Vash operative who had infiltrated Starfleet Security, and carried out through compromised A500 synths originating from the Daystrom Institute's Division of Advanced Synthetic Research.
At some time following the defeat of the Reman military coup of the Romulan Star Empire, it was discovered that the Romulan sun would soon go supernova, likely destroying the planets Romulus and Remus. With the affect on other nearby systems, the pending disaster was anticipated to kill at least nine hundred million Romulans. The empire requested aid from the United Federation of Planets. Admiral Jean-Luc Picard convinced Starfleet to mobilize a massive rescue armada – "ten thousand warp-capable ferries" – to aid in the evacuation efforts, which were assembled over Mars. (PIC: "Remembrance")
Oh, a half-Romulan/half-Vulcan member of the Tal Shiar and its ultra-secretive Zhat Vash cabal, held a gathering of Zhat Vash operatives on the planet Aia. She reminded them of the Admonition, showing them devastation wrought upon the galaxy by artificial lifeforms, warning it would happen again. She selected Mars as an ideal target whose destruction would push the Federation towards a ban of artificial lifeforms. (PIC: "Broken Pieces")
On First Contact Day in 2385, only a skeleton crew remained to work at the center of the rescue fleet, the Utopia Planitia Ship Yards, during the holiday. Multiple A500 androids suddenly went rogue. One android, F8, lowered the deflector shields protecting the shipyards and the planet itself, then hacked the Martian defense net. Once the attack began, F8 slaughtered his coworkers and then committed suicide. (PIC: "Remembrance", "Maps and Legends")
Moments thereafter, numerous synth ships began targeting the rescue armada, the Utopia Planitia Ship Yards, the Mars Orbital Facility, and other Martian facilities. The satellite network surrounding Mars was turned against the surface. The synths then began bombarding the surface, igniting flammable vapors in the stratosphere and causing massive damage to the planet's infrastructure.
Although initial reports only estimated more than three thousand killed, the death toll would eventually be revised to 92,143 in total. During the initial news reports, Admiral Picard was quoted on the Federation News Network, calling the attack "devastating".
Since it was initially thought to be a programming error in the synths, the Federation Council placed a ban on synthetic lifeforms. Androids such as B-4 were dismantled, and the Daystrom Institute was restricted to "theoretical" studies on synthetics. Admiral Picard called this ban "a mistake". With all active positronic matrixes shut down, people who would contract mendaxic neurosclerosis, such as Thaddeus Troi-Riker, would eventually die from the illness without treatment, increasing the death toll beyond the initial attack.
With the destruction of the Romulan rescue armada, Admiral Picard submitted a plan to replace the lost ships with mothballed vessels from Beta Antares Ship Yards and the 40 Eridani A Starfleet Construction Yards. Starfleet, aware that the Romulan rescue mission was unpopular among many of the Federation's member races, and with several saying they would leave the Federation if the rescue went forward, instead abandoned its promise to aid the Romulan evacuation altogether, a decision that shocked Picard.
In desperation, he submitted his resignation, demanding that Starfleet either accept it or honor their promise to the Romulans; to his surprise and dismay, they accepted the resignation, which angered Lieutenant Commander Raffi Musiker, who believed in his plans and was eventually discharged from Starfleet. In the years that followed, Picard developed recurring nightmares of the attack, partly from his loss of faith in Starfleet. Musiker, who had initially believed in some Tal Shiar involvement and not the accepted story of a programming error, would become obsessed with figuring out who really carried out the attack, with her conspiracy theories only getting as far as the Conclave of Eight. This obsession would lead to the destruction of her family and her drug addiction.
In 2387, Ambassador Spock would attempt to stop the supernova from happening by using red matter aboard the starship Jellyfish. He would be too late, and when the Romulan sun did go supernova, it destroyed the Romulan system, killing millions. The Jellyfish would be attacked by the Romulan mining vessel Narada, whose captain Nero blamed Spock and the Federation for the death of his wife in the supernova. Both ships would be lost in a red matter-created black hole, leading to the creation of an alternate reality. Picard would go on to blame Starfleet's inaction for the high death toll, lamenting that their decision to call off the rescue and to abandon the Romulans was not just dishonorable, but also downright criminal, and it was not so easy for those either had died or left behind. The Romulan Star Empire was replaced by a Romulan Free State.
With the lack of Starfleet support, the sectors in and near the former Romulan Neutral Zone collapsed, and the planets there were left to themselves. Independent groups such as the Fenris Rangers would attempt to maintain order and help worlds like Vashti, the Romulan Relocation Hub before the attack, but by the end of the century these worlds would fall into chaos and poverty, becoming hotbeds for the Romulan Rebirth movement and petty warlords like Kar Kantar.
As of 2399, Mars remained ablaze. Some individuals, such as Dr. Agnes Jurati, later referred to the planet as having been "destroyed" in the attack. At the same time, however, the Federation's synthetic ban was eventually lifted after it was discovered that the synthetics were not responsible for the attack, but rather it was orchestrated by a secret group of Tal Shiar operatives known as the Zhat Vash, whose sole mission was the extermination of all synthetic life. (Star Trek; ST: "Children of Mars"; PIC: "Remembrance", "Maps and Legends", "The End is the Beginning", "Absolute Candor", "Stardust City Rag", and more)
In Star Trek Online, a Task Force Operation PVE mission based on the Attack on Mars is known as "Synth Wave". In it, players are tasked with attempting to rescue as many engineers as possible to get them to safety or get them into various Starfleet vessels to fight against the synth armada. Since the players' involvement in the event is only hypothetical, players can see what their actions could do in lessening the death toll of the attack.