Duhaime's Law Dictionary

Declaration of War Definition:

An explicit warning from one state to another, in the form either of a reasoned intent to commence hostilities or of an ultimatum which carries the same result.

Related Terms: War

As there is no international law except to the extent that individual states ratify international treaties, there is no legal requirement that any form of notice of a declaration of war precede hostilities. However, signatories to one of the 1907 Hague Conventions (Opening of Hostilities (Hague III); October 18, 1907) are bound by this:

"Article 1: The Contracting Powers recognize that hostilities between themselves must not commence without previous and explicit warning, in the form either of a reasoned declaration of war or of an ultimatum with conditional declaration of war. "

Emerich de Vattel (1714-1767), in his exquisite law book called The Law of Nations,  proposed these principles of law as regards the declaration of war:

"[T]o declare to that unjust nation, or its chief, that we are at length going to have recourse to that last remedy, and make use of open force, for the purpose of bringing him to reason. This is called declaring war....

"A declaration of war being necessary, as a further effort to terminate the difference without the effusion of blood, by making use of the principle of fear, in order to bring the enemy to more equitable sentiments - it ought, at the same time that it announces our settled resolution of making war, to set forth the reasons which have induced us to take up arms....

"It is necessary that the declaration of war be known to the state against whom it is made.... It is necessary for a nation to publish the declaration of war for the instruction and direction of her own subjects....

"He who is attacked and only wages defensive war, needs not to make any ... declaration - the state of warfare being sufficiently ascertained by the enemy's declaration, or open hostilities....1

"The declaration ... need not be made till the army has reached the frontiers. It is even lawful to delay it till we have entered the enemy's territories and there possessed ourselves of an advantageous post. It must, however, necessarily precede the commission of any act of hostility."

The Romans had a formal process of declaring war on a prospective enemy. First, they sent an ambassador to set out the grievance and demand redress within 33 Days. Failing satisfaction, another ambassador was sent to herald war (not a gig any life insurance broker would be enthusiastic about).

England's September 3, 1939 declration of war against Germany was worded as follows:

"It is notified that a state of war exists between His Majesty and Germany as from 11 o'clock A.M., today, the 3rd September, 1939."

Failing a declaration of war, a state's hostile actions against another may be construed as piracy. 2

The state of war is not a matter for the Courts, but for the Government. Note in this regard the words of Justice Grant of the English Court of Appeal in The Pelican case of 1809:

"It always belongs to the Government of the country to determine in what relation any other country stands towards it. That is a point upon which courts of justice cannot decide."


  • NOTE 1: but see remarks by Justice Stowell in The Eliza Ann, 1 Dods. 244 at page 246 (1813) in which he suggests that a unilateral declaration may not trigger a state of war.
  • NOTE 2: McNair, Arnold Duncan, Legal Effects of War (Cambridge: University Press, 1948), page 4
  • The Pelican, Edw. iv (1809). Approved in Blackburne v Thomson 15 East 81 at 90 (1812)
  • Vattel, Emerich de, The Law of Nations, translation Joseph Chitty (Philadelphia: T. & J. W. Johnson Law Booksellers, 1839)

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