Dulce is the name of a city in New Mexico (USA) where, near the Apache Reservation, on the Archeluta Mesa, the US military has a secret facility.
As a result of the Edwards Agreement (see: Edwards Agreement) mixed bases, with alien and human staff, were established, most of them in underground complexes. Dulce is one ot them. Phil Schneider claimed to have assisted in its construction.
In these facilities, human and alien scientists would co-operate and exchange information. The alien "ambassadors" were also allocated quarters there. And, more sinisterly, some of these bases were (and still are) used to collect abductees.
The co-operation between the humans and the (mainly reptilian) aliens did not go smoothly. Often there were tensions because of cultural differences between the reptilian aliens and the human staff (who are less disciplined and far more curious than the reptilian staff). On more than one occasion, human soldiers got arrested or simply disappeared after accessing restricted areas. Things got out of hand in the Dulce facility where the aliens ended up refusing humans access to the lower levels of the base. At a given moment, around 1979-1980, it is said that it actually came to a fight in which dozens died. Top-level interventions from both sides were needed to prevent the situation from escalating.

Some authors, with links to the intelligence community, claim that the incident in Dulce never happened, and that it was part of a disinformation campaign. Then again, such denial might just as well be disinformation.