On this day in 1775, what would become the US postal service was established by Benjamin Franklin. He envisioned a mass communication network across the colonies. After the Declaration of Independence this communication was vital to the cause of the American Revolution. Now it's a way for bill collectors to get their message across, "Some guys are coming to your door soon".
On this day in 1908 the Federal Bureau of Investigation is founded. It became the chief means of the government to investigate federal crimes. Throughout its history, the FBI was able to nab criminals who had otherwise eluded state agencies, whether because of lack of evidence or just plain corruption. J. Edgar Hoover became the head of the FBI in 1924, just 7 years after joining the department. He came to symbolize the US government agent tracking down the career criminal during prohibition. Al Capone, "Machine Gun" Kelly, "Babyface" Nelson, and John Dillinger were just a few of those brought to justice under his administration.
On this day in 1941 the US froze all Japanese assets after Japan invaded French Indo-China, present day Vietnam. This was just another step towards war in the Pacific, and yet another reason to call into question the US government claim of having "no clue" an attack was imminent at Pearl Harbor.
On this day in 1948 President Harry Truman signed Executive Order 9981 officially desegregating the US military. Blacks had fought in every major American conflict, but often times they were forced to do the job that white soldiers wouldn't. Apparently WWII changed enough minds that this order was presented to Truman and he was progressive enough to sign it.