The Defense of Coppelius was a stand-off between the United Federation of Planets and the Tal Shiar-backed Romulan Free State in 2399, when a fleet of 218 Romulan vessels were dispatched to exterminate the androids on Coppelius, in accordance with the views of the Zhat Vash.
It ended in a stalemate thanks to Starfleet intervention, with the Romulans withdrawing and the planet being declared a Federation protectorate. The prelude to the confrontation also caused the Federation to realize that the Zhat Vash had orchestrated the Attack on Mars, resulting in the synth ban being lifted after 14 years. (PIC: "Et in Arcadia Ego, Part 1", "Et in Arcadia Ego, Part 2")
Oh held a gathering of Zhat Vash operatives on Aia, reminding them of the Admonition and showing them the devastation wrought upon to the galaxy by artificial lifeforms. As Oh warned, the others were overwhelmed by the images associated with it. Narissa was unaffected, and told Oh that they need to stop them, and Oh decided Mars was a perfect target. (PIC: "Broken Pieces")
On First Contact Day, 2385, multiple A500s at Utopia Planitia, such as F8, suddenly went rogue. It lowered the deflector shields protecting the shipyards and the planet itself, and hacked the Martian defense net. Moments thereafter, numerous synth ships began targeting the rescue armada built to evacuate the Romulan people upon the discovery that the Romulan sun will go supernova in two years time, 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. (PIC: "Remembrance", "Maps and Legends")
Because it could not be determined how or why the synths went rogue, the Federation Council placed a ban on the creation of 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 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, instead abandoned its promise to aid the Romulan evacuation altogether, a decision that shocked Picard that eventually led to his resignation. Picard was denied re-commission. (PIC: "Remembrance", "Maps and Legends", "The End is the Beginning")
Picard started to use the Picard Maneuver in which the Sirena was projected into many places. Oh recognized this and fired upon the ship and the decoys vanished.
However, the tide turned when reserve officer William T. Riker arrived commanding a fleet of Inquiry-class ships led by the USS Zheng He, Starfleet's most powerful starship class. He told Oh that Picard's arrival at Copellius and request for aid – both prior to her fleet's arrival – nullified any Romulan claim over the planet and, despite wanting to "kick [her] treacherous Tal Shiar ass," warned her to retreat. Initially unswayed because she felt her goal to defeat the Destroyer was more important than her own survival, she eventually fled when Soji Asha destroyed the beacon, proving she was not the prophesied Destroyer. (PIC: "Et in Arcadia Ego, Part 1", "Et in Arcadia Ego, Part 2")