Nechayev blasted Picard for his decision to return the Borg known as Hugh to the Borg collective without also sending the invasive program developed by the Enterprise-D personnel. Picard protested that using the program, effectively a genocidal weapon of mass destruction, would be a violation of Starfleet's principles, but Nechayev, frearing a mass invasion by the Borg, ordered him to use the weapon if another opportunity presented itself. ("Descent, Part I")

from the Star Trek Encyclopedia

Editor's Note: Nechayev has also had dealings with the DS9 crew, as well as additional dealings with the Enterprise crew