Understanding why the Navajo code proved so resistant to Japanese analysis requires some understanding of what Imperial Japanese signals intelligence was actually capable of — which was, in many respects, considerable.

Japanese military intelligence had invested significantly in signals intelligence capabilities throughout the 1930s and into the war.

They had broken or partially compromised several American and Allied codes during the early years of the conflict, and their ability to monitor and analyze American radio traffic was a genuine concern for American commanders throughout the Pacific campaign.

Their cryptanalysts were capable of working with multiple language systems.

Japan had historically engaged with Chinese, Korean, Russian, German, and English codes and had analysts with expertise in each.

The European and Asian languages that formed the basis of most Allied codes were, to varying degrees, accessible to Japanese analysis.