Nomenclature for US government entities can be complicated. Here are some guidelines about how to style names of administrative jurisdictions, departments, and agencies:
The formal style for Cabinet-level departments is “the Department of State,” for example, though journalistic style often up-ends this form as, for instance, “the State Department.” Informally, a department may simply be called “State” or “Interior” or “Justice.” Some departments are also recognized by their initials (DOJ for “Department of Justice,” for example), though abbreviations should be used only on second reference (an editing term that actually means “all subsequent references”), after the name is spelled out the first time it is used.
Abbreviated forms of names should be preceded by the (“the DOJ,” for example), unless the abbreviation is an acronym (pronounced as a word), such as OSHA (the abbreviation for the Occupational Safety and Health Administration), which is pronounced “oh-shuh,” not “(the) oh-es-aitch-ay.”
Be sure, too, that identification is unambiguous. Two Cabinet-level departments — the Department of Education and the Department of Energy — can be abbreviated DOE, so if both departments are mentioned in a particular article or book, it’s best not to use the abbreviation.
Also, the designation US often precedes a department or agency name to distinguish it from a state-level entity of the same name or a similar foreign entity, whether such an entity is mentioned in the same piece of content or not. (Note that many publications and Web sites continue to use initial periods in US, but the latest edition of The Chicago Manual of Style advises that periods now be omitted from the abbreviation.)
When an entity is referred to generically, even if the term is part of the entity’s name, the wording should, by definition, be lowercase: “the department,” “the bureau” (for the Federal Bureau of Investigation, for example), “the postal service” (in reference to the US Postal Service). Entities themselves often capitalize such shorthand, but that doesn’t mean you have to.
Beware of new nomenclature: The agency long known as the US Immigration and Naturalization Service was dismantled and most of its functions and responsibilities taken up by the newly created agency US Citizenship and Immigration Services in 2003. In nonfiction referring to immigration before that date, this information should be given to inform readers of the distinction; in fiction set before that year, the former agency, not its successor, should be mentioned in references to the US government’s immigration policies.
Likewise, what has been known as the US Centers for Disease Control and Prevention (but is still abbreviated CDC) for twenty years has undergone half a dozen name changes since the Office of National Defense Malaria Control Activities was established in 1942.
These are only two examples illustrating that writers should take care to identify government entities according to the historical context in which their articles and books take place.
When it comes to identifying government entities, due diligent research to make sure your usage is accurate. Don’t let your writing be merely, as the saying goes, good enough for government work.