A bombing by a Changeling infiltrator at a diplomatic conference in Antwerp in 2372, which killed twenty-seven people and caused a security panic on Earth.
The Antwerp Conference bombing is the event that begins the Deep Space Nine two-part story Homefront and Paradise Lost. A conference between Federation and Romulan delegates in Antwerp is destroyed by an explosive device, and recordings show a Changeling at the scene. It is described as the worst such attack on Earth in about a hundred years.
Benjamin Sisko and Odo are called to Starfleet Headquarters, where Admiral Leyton makes Sisko acting head of Starfleet Security. Blood tests and phaser sweeps are brought in, and when the planet's power grid fails the Federation president reluctantly declares a state of emergency and puts armed officers on the streets.
Sisko later finds that the power failure was the work of Leyton's own cadets and that the admiral means to replace civilian rule. A Changeling tells Sisko that only four of its kind are on Earth, and that fear has done the rest.