1. To soothe anger or agitation; synonyms: mollify, conciliate, appease, placate.
2. To subdue by armed action; reducing to a state of submission; especially, by military force; subduing.
3. To fight violence and to make efforts to establish peace in an area.
4. Bringing or restoring to a state of peace or tranquility; encouraging quiet; making calm: "She made every effort at pacifying the angry man."
5. Appeasing; such as, pacifying one's appetite.

Unhappy babies are often given a rubber device for sucking called a pacifier to stop their crying. In the same way, someone stirred up by anger or some other strong emotion can usually be pacified by resolving or removing its causes.

In a usage that refers to a military combat area, it means using armed forces to neutralize the enemy there and to quiet the local people who may have been supporting them.