Defense of Coppelius
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. ()

Prelude 
Oh held a gathering of Zhat Vash operatives on Aia, reminding them of the Admonition and showing them the devastation wrought upon the galaxy by artificial lifeforms, led by a mythical Destroyer. 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. ()

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. ()

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. ()

After discovering that Bruce Maddox had been creating more synths in secret, Oh ordered her operatives to find them and force them to divulge the location of their home. Narissa attempted to capture Dahj Asha, who was living in Boston at the time, believing herself to be human, but she failed and Dahj died before she could give the Romulans any information. However, Narissa's brother Narek managed to manipulate Soji Asha, who also believed she was human and worked for the Borg Reclamation Project at the time, into revealing her planet of origin. () 

Abandoning her Starfleet cover, Oh led a Tal Shiar  squadron heading to Coppelius to destroy the synths living there. () 

Picard and the crew of La Sirena managed to arrive at Coppelius before Oh's armada and warn the synths of the impeding attack. To protect her people, the android Sutra deciphered the message hidden in the Admonition, realizing it was a warning left for sentient androids by an extra-galactic alliance of synthetic life. It contained subspace frequencies needed to contact these powerful beings and allowing them to open a portal into their space and wipe out all organic life that might pose a threat to the synths who contacted them. Sutra and Soong designed a beacon that could transmit the frequency and the synths began building it with Soji's help, despite all efforts by Picard and his crew to stop them. ()

The Battle 
When the Romulan armada arrived over Coppelius, the androids sent out 23 orchids, a form of orbital defense weapon, intended to take out smaller ships. While the orchids were able to stall the Romulans for a while and even destroyed several Warbirds, they were no match for the number and firepower of the fleet. 

Back on the surface, Dr. Agnes Jurati freed Picard from the house arrest the synths had placed him under, and the two of them managed to launch La Sirena. While Picard had no hope of defeating the Zhat Vash, his plan was to stall them until the backup he had requested from Starfleet could arrive. Additionally, he hoped that in sacrificing his own life, he might inspire Soji and the other synths to trust that not all organic beings were their enemies. 

After the orchids were destroyed, Picard and Jurati maneuvered La Sirena in between the Romulan armada and the planet. General Oh was poised to ignore them and sterilize the planet, but Jurati enacted a version of the Picard Maneuver to buy more time. Using the fundamental field replicator created by the Coppelius androids, she conjured many images of Sirena projected into space, each with its own warp signature, to fool the Romulan sensors. The ruse succeeded and the warbirds began engaging the projected ships. However, as soon as a shot hit the real Sirena, the illusion was disrupted, and the false ships disappeared. 

While Picard's strategy briefly interrupted Soji's progress in building the beacon, she finished her work as soon as La Sirena was hit. The beacon sent out an energy pulse which opened a portal in space, allowing the extra-galactic synths to reach through. 

General Oh ordered her soldiers to prepare once again to eliminate any life, organic or synthetic, on Coppelius. However, moments before she could give the order to fire,  reserve officer William T. Riker arrived commanding a fleet of  ships led by the , 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. 

Oh remained unswayed, since she felt her goal to defeat the Destroyer – and thus potentially save all organic life in the galaxy from destruction – was more important than her own survival. 

On La Sirena, Picard realized he was about to die from his illness. He asked Jurati to give him stimulant medication that might hasten his death but would allow him to remain clear-headed until then. On an open channel, Picard pleaded with Soji to deactivate the beacon and prove to the Zhat Vash that she was not the Destroyer. Finally swayed, Soji destroyed the beacon and the portal closed again, forcing the extra-galactic synths back into their own realm. 

Finally, Oh ordered her troops to stand down and they were escorted back to Romulan space by Riker's fleet. 

Moments later, Picard's body started shutting down and Soji beamed him and Jurati down to Coppelius, where Picard died. ()

Aftermath
Dr. Jurati and Soji managed to transfer Picard's consciousness into an artificial body with the help of Dr. Altan Inigo Soong, thus effectively resurrecting him as a sentient synth.

As a consequence of the events during the Defense of Coppelius and the revelations about Oh and the Attack on Mars, the ban on synthetic lifeforms was repealed in 2399. ()

Conflicts