I've been reading the Constitution, and I noticed an interesting point: the only federal law enforcement authorized by the Constitution was the state militias, which at the time consisted of all the able-bodied men of each state, and was commanded by officers from that State. This provided a check on the government that we have given up: how comfortable could the federal government be asking a non-professional group…
Other wars before these played a role but I feel like WWII was the turning point after which the US was a different place. We entered more or less a permanent state of war or war-readiness since then, and the Cold War provided a rationale to massively increase the size of our espionage apparatus. Originally the idea was that this spy apparatus would only be aimed outward, but that was always a fantasy. Anything we aim outward will eventually be aimed inward.