Declaration of War required:
The contracting parties recognise 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 accompanied by a conditional declaration of war.
The framers of the Hague Rules were agreed to one rule, namely, that an attack which nothing foreshadowed would be Infamous. A gross violation of International law would be committed by the commencement of hostilities in time of peace without a previous controversy and negotiations with a view to a peaceful settlement. (Vide Hague Peace Conferences, Higgins, p.203).
Is Surprise Still Possible?
Nothing in the foregoing rule requires that any time shall elapse between the actual declaration of war and the commencement of hostilities. It is still possible, therefore, to make a sudden and unexpected declaration of war and thus surprise an unprepared enemy.
The French proposal to The Hague Peace Conference of 1907, based substantially on resolutions of the Inst. International Law at Ghent in September 1906 consisted of three articles. The first two were embodied substantially as in the text above, while the third “Hostilities should not begin till after the expiry of a delay sufficient to insure that the rule of previous and unequivocal notice may not be considered as
evaded,” was rejected.
What about Neutrals?
The existence of a state of war must be notified to the neutral powers without delay, but shall not take effect in regard to them until after the receipt of a notification, which may, however, be given by telegraph. Neutral powers, nevertheless, can not rely on the absence of notification if it is clearly established that they were in fact aware of the existence of a state of war.
Is it Legally Binding between Parties?
The present convention shall take effect in case of war between two or more of the contracting Powers. Article II is binding as between a belligerent Power which is a party to the convention and neutral powers which are also parties to the convention.
Legal and Commercial Importance
This convention is important from both the legal and commercial point of view since it requires belligerents themselves to publicly announce a definite date for the commencement of hostilities, from which date they become entitled to exercise the rights of belligerency, and are themselves required to comply with and to exact from neutrals the obligations of neutrality.
Hague Convention III: Articles I to III.
US Army. (1914) Rules of Land Warfare. Available from World Wide Web: http://www.loc.gov/rr/frd/Military_Law/pdf/rules_warfare-1914.pdf. [Accessed: 28 September, 2015].