Beachhead
A beachhead is a military term used to describe the line created when a unit (by sea) reaches a beach, and begins to defend that area of beach, while other reinforcements (hopefully) help out, until a unit large enough to begin advancing has arrived. Beachheads were very important in operations such as D-Day, the Korean War (especially at Inchon), and the Vietnam War, among many other examples.
See also