I'm not sure the government is actually authorized to spy, per the US Constitution. It seems that it (spying) would only be authorized as part of a military operation.
Would you like to revive George Washington and tell his ghost that his spy rings were unconstitutional?
However, the Constitution does not grant Congress or the Courts to spy, or to hire spies, or direct spies, or have anything to do with spies.
Spying and intelligence are still, in the present day, seen as a matter of national defense. NSA is explicitly under the Department of Defense, for example. CIA is civilian, but is still framed as serving a defense/national security purpose (thus Constitutionally justifiable) and scoped to be foreign-facing (domestic intelligence is primarily the FBI's province, "necessary and proper" for enforcing federal law).
Using spies (human intelligence) is as basic to warfare and defense as any other weapon.
Spying on citizens is where things get murky.
And as much as many readers here may dislike the intelligence agencies, distinguishing between domestic and foreign targets in the internet age is not an easy problem to solve.
Especially considering they're damned when they go too far (e.g. Prism) and damned when they don't (9/11).
Computers, code books, secure phones, etc. are required for communication.
Sometimes, more interesting things get sent. https://en.wikipedia.org/wiki/Diplomatic_bag#Noteworthy_ship...
Paragraph 2.1 states "2.1 Need. Accurate and timely information about the capabilities, intentions and activities of foreign powers, organizations, or persons and their agents is essential to informed decisionmaking in the areas of national defense and foreign relations. Collection of such information is a priority objective and will be pursued in a vigorous, innovative and responsible manner that is consistent with the Constitution and applicable law and respectful of the principles upon which the United States was founded."
The CIA and NSA would be surprised to hear that.