The 2399 crisis on the synthetic colony of Coppelius, where a Romulan fleet, a Starfleet fleet and a call to ancient machine beings nearly ended in disaster.
Coppelius was the hidden home of a community of synthetic beings created by Altan Inigo Soong and the descendants of Data's work. When Jean-Luc Picard brought Soji there in 2399, the colony learned that a Romulan fleet led by Commodore Oh was coming to destroy it.
Soji's sister Sutra persuaded many of the synthetics to build a beacon that would summon a powerful alliance of synthetic life, which would wipe out organic life to protect them. Picard tried to stall the Romulans in La Sirena while the beacon's portal began to open.
Riker arrived with a Starfleet fleet led by the USS Zheng He to defend the colony. Picard then persuaded Soji to shut the beacon down, and Oh's fleet withdrew. The standoff ended without a full battle, but Picard's long-standing illness caught up with him and he died. His mind was then moved into a new synthetic body.