Janeway is warned off from having the Dauntless enter the Neutral Zone by a set of Romulan Warbirds that decloak in front of her. The purpose of the Neutral Zone is to be, as the name implies, a section of space that is neutral territory, to act as a buffer between Federation space and the Romulan empire. Neither Starfleet nor Romulan ships are allowed to enter the Neutral Zone without express consent from both governments. The ships that warned the Dauntless off were themselves in violation of the Federation/Romulan treaty that established it.