When and how do foreign policy bureaucracies speak with one voice? Applying measures from linguistics to a novel corpus of all President’s Daily Briefs (PDBs) from 1961 to 1977, this article describes and explains the increasingly standardized and technical writing in a key intelligence bureaucracy. Assessing 5,000 intelligence briefs featuring over 4 million words, we use novel measures of text-at-scale and find a steady decline in lexical diversity and an increase in lexical complexity over time, regardless of leader in power and exogenous changes in foreign policy events. Such changes, in contrast, are absent in the same time period in corpora of State Department materials, presidential speeches, and New York Times articles. We explain this pattern with a theory of the increasing bureaucratization of writing. Increasing specialization (leading to greater organizational complexity) joined by increasing centralization of communication outputs (asserting more organizational control over writing and speaking) leads a bureaucracy to speak with a more singular and rational-technical voice. We find quantitative and qualitative evidence of this process during the 1960s and 1970s. The article contributes by shedding light on bureaucratic emergence. While past scholarship has debated the consequences of mature bureaucracy on foreign policy, this article explains when and how bureaucracies speak with one voice in the first place. It also shows the promise of analyzing unique corpora of declassified documents with innovative techniques that identify linguistic patterns at scale.