beachhead -
an initial accomplishment that opens the way for further developments; "the town became a beachhead in the campaign to ban smoking outdoors"; "they are presently attempting to gain a foothold in the Russian market"
beachhead -
a bridgehead on the enemy's shoreline seized by an amphibious operation; "the Germans were desperately trying to contain the Anzio beachhead"
beachhead -
An initial success that ensures the possibility of further advances in a project; a foothold.
Wikipedia
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 (it is hoped) help out, until a unit large enough to begin advancing has arrived. Sometimes used interchangeably (both correctly and incorrectly) with Bridgehead and Lodgement. Beachheads were very important in operations such as Operation Neptune, the Korean War, and the Vietnam War, among many other examples.