beachhead

A beachhead is a temporary line created when a military unit reaches a landing beach by sea and begins to defend the area while other reinforcements help out until a unit large enough to begin advancing has arrived. The term is sometimes used interchangeably (both correctly and incorrectly) with bridgehead and lodgement. Beachheads were very important in operations such as Operation Neptune during World War II, the Korean War (especially at Inchon), and the Vietnam War, among many other examples.