During the First World War, the Netherlands maintained a stance of carefully guarded neutrality. International tele communications in the form of telephone and telegraph were closely monitored and censored by so-called censorbureaus. In 2019 new files were declassified and released to the Dutch National Archive about the secensorship bureaus at Amsterdam and Rotterdam, covering 1914 to 1918. They provide detailed insight in the day-to-day business, the codebreaking efforts and specific cryptanalytic results. The material provides a completely new perspective on the genesis of modern Dutch codebreaking. This article gives a first survey of the development of these interception bureaus. It analyses their pioneering codebreaking activities and presents historic material on German diplomatic ciphers. Also, it provides new insight into the mysterious sale in 1919 of German codebooks from the Netherlands to the United States, as reported earlier in the literature.