Today's gem is that in 1853, the United States asked its diplomats not to wear court dress any longer. Wikipedia's explanation is a bit more involved:
...In 1853, Secretary of State William L. Marcy issued a circular recommending that U.S. diplomats wear “the simple dress of an American citizen.”It goes on to state that modified Navy uniforms were in use for a while, but the practice was stopped by FDR in 1937, and codified in law in 1946.
In response to what was perceived as the excessive ostentatiousness of some of these individualized uniforms, Congress banned diplomatic uniforms altogether in 1867, by passing a resolution forbidding diplomatic officials to wear "any uniform or official costume not previously authorized by Congress". This caused some discomfort to American diplomats, who now had to appear "underdressed", in evening dress, to official functions. In 1910, Theodore Roosevelt attracted considerable attention when he was the only foreign official at the funeral of King Edward VII who was not in uniform.
Now... it's pretty clear the intent of the law is being followed. But a quick search of pictures of diplomatic staff suggests "black suit and tie" is a de facto uniform for males. (A few grey suits, I admit.) How much uniformity is too much? Are members of other departments also forbidden to wear uniforms unless authorized by Congress? Who would have standing to sue if the State Department violated this law?