Concerned about large-scale ship-building programs in the US, Great Britain,
and Japan following World War I, US President Warren G. Harding organized
the Washington Naval Conference in late 1921. The conference was held in
Washington DC and nine nations attended. Harding's goal was to establish
limits on the number and tonnage of warships of each nation. In addition to
concerns of a naval arms race, Harding and other nations were concerned
about Japanese expansion in the Pacific.

Herbert Yardley's "Black Chamber," a civilian cryptography bureau funded
jointly by the State Department and the US Army, intercepted and deciphered
the communications of the participating countries' delegates. Over the
course of a few months, Yardley's team deciphered or translated nearly 5,000
messages. Notably, shortly before the conference, Yardley's team had a
major success when it broke the Japanese diplomatic code and learned of the
Japanese government's instructions to its ambassadors. Naturally, knowing
the lowest naval ratio the Japanese were willing to accept put the US in a
powerful negotiating position.

The Washington Naval Conference is hailed as the first disarmament
conference in history and a successful one as well. The resulting treaties
remained in effect until 1936 when Japan backed out. The role of the Black
Chamber during the conference also had a significant impact on the field of
signals intelligence. The Black Chamber enjoyed increased funding and
support in the years following, and its chief, Herbert Yardley, received the
Distinguished Service Medal. This support lasted until the Black Chamber
was closed by Secretary of State Henry Stimson, in 1929.