Technically, the statement by a governing body (or authorized spokesperson thereof) of a sovereign nation
that a state of war
now exists (or will exist) between said sovereign nation and another sovereign national body. First codified
in the modern 'national' sense in the Magna Carta
The actual declaration of war serves a myriad of purposes, many having to do with international law and more having to do with domestic law.
Some of these include but are not limited to:
- Military attacks by the declarer on the forces or territories of the declared target may occur without prior warning
- All previous agreements between the two nations in the international legal arena are null and void
- Citizens of the other nation found within the declaring nation may be detained or deported from the declaring nation without warning or further reason
- The declaring nation warns all other nations that military operations against the target may occur and that non-involved military or civilian personnel and units of other nations should take all possible steps to identify themselves as such and leave war zones.
These are just a few of the results that may arise. On the domestic law front, I'll stick to the U.S. as my example. In time of declared war, the authority of the U.S. President to command the conduct of military operations and forces is made absolute; Congress 'cedes' the warfighting authority to the Executive Branch and its designated military subordinates for the duration of the conflict. Note that the Joint Chiefs of Staff, while high-ranking military officers all, are not in the direct chain of command. This runs from the President to the Unified Commanders (at least, operational command does) such as CINCCENT, SACEUR, CINCLANT, etc.