The most impressive? it isnt even on the list.
A military doesnt just go to the slaughter, although it can. In actuality, there are two civil engineering achievments directly inspired and performed by units of the US Military. and yes both are defense related.
The first is the finishing of the Panama Canal. If it wasnt for the public health aspects and disease control efforts of the US Army Medical Corp and Col George Washington Gobbels that ditch would have never been completed. Remember, the French had the plan, and used a lot of men only to fall to failure because of disease and engineering problems were too much. The US Army came through by conquering the yellow fever mosquito and its breeding areas, enabling the big ditch to continue to completion.
The second item was the building of the ALCAN Highway, from Alaska to the US through Canada, some say the further extension of the road to south America really completed the project. read about the ALCAN sometime, or better yet, drive it. it is a totally nerve wracking experience.