The Army’s encryption systems, which used mechanical cipher machines and required trained operators to encode and decode messages, had been compromised at various points.

Even when the underlying code was not broken, traffic analysis — the study of the patterns of communications without necessarily understanding the content — could reveal the size, movement, and timing of American operations.

The encryption systems in use were also slow.

A single message of any significant length could take an encrypted-transmission specialist thirty minutes or more to prepare, transmit, and decode on the receiving end.

In the dynamic, fluid conditions of amphibious landings and jungle fighting, thirty minutes was an eternity.

A Marine battalion commander who needed to direct artillery fire on a position that was pinning down his men could not wait half an hour for the fire mission to travel through an encryption system.