beachhead

Etymology

From beach + head.

noun

1. (military) An area of hostile territory (especially on a beach) that, when captured, serves for the continuous landing (or movement into position) of further troops and material
2. (by extension) A coastal landing place.
   … the SR is confident of Newhaven's appeal as a starting point convenient for Southern England generally and of Dieppe's attraction as a beachhead from which motorists' routes to the principal holiday areas from the French coastline will be shortened. 1964 July, “BR develop Newhaven as UK's chief car-ferry port”, in Modern Railways, page 48
3. (by extension) An initial success that ensures the possibility of further advances in a project; a foothold.
   Muslim influence in Arakan was of great cultural and political importance. In effect, Arakan was the beachhead for Muslim penetration into other parts of Burma even if it never achieved the same degree of importance it did in Arakan. As a result of the close land and sea contacts maintained between the two countries, Muslims played a key role in the history of the Kingdom of Arakan. 2002, Moshe Yegar, chapter 2, in Between Integration and Secession: The Muslim Communities of the Southern Philippines, Southern Thailand, and Western Burma/Myanmar