Quoting directly from the reference: "The objectives of this problem included the training of commanders in estimating he situation and planning, scouting and screening, convoy escort, communications, advanacead base operations, and coordination of various types of platforms, as well as to study varioud fleet and carrier task forace defensive formations, including ASW defense, fleet tactics against air attack from carier and land-based aircraft. PBY swarm attack tactics, and offensive and defensive night destroyer tactics, as well as underway refueling of all types of ships."
Another direct quote: "In practical terms, there were two different operational questions examined in Fleet Problem XXI. Part II focused on the operations of a concentrated fleet (Japan) seeking to disrupt the preparations of a dispersed one (United States) during the opening days of a Pacific War, while Part VI involved a weaker fleet (United States) trying to counter a stronger one (Japan) attempting to capture its main advanced base."
Part III took place in the vicinity of the Lahaina anchorage (also known as "Lahaina Roads") shown in the postcard above. Destroyer Underway Refueling, Cruiser/Destroyer Tactics and Anti-Air & ASW/PBY-Submarine Joint Attack were the titles of the excercises in this part.
In Part IV, the fleet was resupplied and refueled at Lahaina anchorage. The 4/12/40 date on the photo shows that it was taken during this part. In Part V the ships began to take their positions for Part VI in which Lahaina was the invasion target.
- Nofi, "To Train The Fleet For War, The U.S. Navy Fleet Problems 1923-1940", U.S. Gov. Official Edition