John Locke

Men living together according to reason, without a common superior on earth, with authority to judge between them, is properly the state of nature. But force, or a declared design of force, upon the person of another, where there is no common superior on earth to appeal to for relief, is the state of war: and it is the want of such an appeal gives a man the right of war even against an aggressor, tho’ he be in society and a fellow subject. Thus a thief, whom I cannot harm, but by appeal to the law, for having stolen all that I am worth, I may kill, when he sets on me to rob me but of my horse or coat; because the law, which was made for my preservation, where it cannot interpose to secure my life from present force, which, if lost, is capable of no reparation, permits me my own defence, and the right of war, a liberty to kill the aggressor, because the aggressor allows not time to appeal to our common judge, nor the decision of the law, for remedy in a case where the mischief may be irreparable. Want of a common judge with authority, puts all men in a state of nature: force without right, upon a man’s person, makes a state of war, both where there is, and is not, a common judge.

John Locke, from the Second Treatise on Civil Government (Thomas Hollis Edition. [London: A. Millar et. al.] 1764), Chapter Three: “Of the State of War.”