Complex sentence: Deciding they could not let the Borg assimilate all of the Alpha Quadrant, Starfleet Command successfully seized control of a transwarp conduit, in an attempt to launch a counteroffensive, lead by Captain Picard, into the Delta Quadrant, the home of Borg space.
Split into simpler sentences: Deciding they could not let the Borg assimilate all of the Alpha Quadrant, Starfleet Command successfully seized control of a transwarp conduit. Through this, Starfleet attempts to launch a counteroffensive, lead by Captain Picard, into the Delta Quadrant, the heart of Borg space.