That's not strictly true. The recent wars weren't "declarations of war", but most (not all) were authorized by Congress. Congress voted on military action against those countries and it passed both chambers, but they didn't "declare war". To be honest I don't understand the difference, other than the words used.
Here are the possibly relevant enumerated powers of Congress:
- define and punish Piracies and Felonies committed on the high Seas, and Offenses against the Law of Nations
- declare War
- grant Letters of Marque and Reprisal
- make Rules concerning Captures on Land and Water
- raise and support Armies
- provide and maintain a Navy
- make Rules for the Government and Regulation of the land and naval Forces
- provide for calling forth the Militia to execute the Laws of the Union, suppress Insurrections and repel Invasions
- provide for organizing, arming, and disciplining, the Militia, and for governing such Part of them as may be employed in the Service of the United States
Nowhere in that list is "for an indefinite period of time, kill a bunch of unidentified people in unspecified foreign lands for unspecified reasons". Wars fought among nations are generally bad things, but historically there has been a protocol that the Constitution references in the simple phrase "declare war". Various acts of violence against various parties for various purposes have been considered necessary for the peace and security of any political unit, but those acts have been limited to the territory of that political unit.
Ever since the USA army wasn't properly stood down from WWII, the requirements of the military-industrial complex have dictated the invention of another procedure for killing thousands of innocents every week on an ongoing basis. A cursory examination of the Constitution reveals that this new innovation violates the enumerated powers of Congress and the executive.
