IR - One Definition A Day
Act of war
Any act which is incompatible with a state of peace. Under customary international law states had the right to resort to war whenever they deemed it necessary. The principal restraint upon this behaviour was thus the laws of warfare. Distinction must immediately be made between the laws covering the conduct of war - jus in bello - and the laws governing the resort to conflict - jus ad bellum. The idea of an act of war, therefore, properly comes under jus ad bellum.
Before the establishment of universal international institutions in the twentieth century, there was a good deal of auto-interpretation attached to this concept. In practice, states could decide for themselves what constituted an act of war. Once war had been declared between the parties then notice was served upon the whole state system that relations had changed from peace to war.
A complicating factor in this was the alliance. States entering alliances took upon themselves obligations to fight each other's wars. If the alliance was to function properly the parties needed to know what constituted an act of war against themselves whereby the alliance would become operational. This is referred to as the casus foederis.
The current century has seen important changes in the laws of war, both ad bellum and in bello. Treaty law, such as that set out in the United Nations Charter, now draws a clear distinction between the legal and illegal use of force. The presumption is now made that force can only be used in self-defence. In the absence of more effective means of conflict resolution, states will resort to force.
The twentieth century has required its statements to be more imaginative in seeking justification for doing so than in the past. At the same time, use of less direct modes of aggression, such as guerrilla warfare, have made it more difficult to apply the laws of war.
External intervention in civil wars has become widespread in the twentieth century. Some of the most intractable regional conflicts - such as the Arab-Israeli conflict - originated as communal differences. In sum, just as international lawyers have attempted to establish new criteria for the use of force, other developments have increased uncertainties belligerency.
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