The state of war is a state of enmity and destruction: and therefore declaring by word or action, not a passionate and hasty, but a sedate settled design upon another man’s life, puts him in a state of war with him against whom he has declared such an intention, and so has exposed his life to the other’s power to be taken away by him, or any one that joins with him in his defence, and espouses his quarrel; it being reasonable and just, I should have a right to destroy that which threatens me with destruction: for, by the fundamental law of nature, man being to be preserved as much as possible, when all cannot be preserved, the safety of the innocent is to be preferred: and one may destroy a man who makes war upon him, or has discovered an enmity to his being, for the same reason that he may kill a wolf or a lion; because such men are not under the ties of the common-law of reason, have no other rule, but that of force and violence, and so may be treated as beasts of prey, those dangerous and noxious creatures, that will be sure to destroy him whenever he falls into their power.
And hence it is, that he who attempts to get another man into his absolute power, does thereby put himself into a state of war with him; it being to be understood as a declaration of a design upon his life: for I have reason to conclude, that he who would get me into his power without my consent, would use me as he pleased when he had got me there, and destroy me too when he had a fancy to it; for no body can desire to have me in his absolute power, unless it be to compel me by force to that which is against the right of my freedom, i. e. make me a slave. To be free from such force is the only security of my preservation; and reason bids me look on him, as an enemy to my preservation, who would take away that freedom which is the fence to it; so that he who makes an attempt to enslave me, thereby puts himself into a state of war with me. He that, in the state of nature, would take away the freedom that belongs to any one in that state, must necessarily be supposed to have a design to take away every thing else, that freedom being the foundation of all the rest; as he that, in the state of society, would take away the freedom belonging to those of that society or common-wealth, must be supposed to design to take away from them every thing else, and so be looked on as in a state of war.
This makes it lawful for a man to kill a thief, who has not in the least hurt him, nor declared any design upon his life, any farther than, by the use of force, so to get him in his power, as to take away his money, or what he pleases, from him; because using force, where he has no right, to get me into his power, let his pretence be what it will, I have no reason to suppose, that he, who would take away my liberty, would not, when he had me in his power, take away every thing else. And therefore it is lawful for me to treat him as one who has put himself into a state of war with me, i. e. kill him if I can; for to that hazard does he justly expose himself, whoever introduces a state of war, and is aggressor in it.
And here we have the plain difference between the state of nature and the state of war, which however some men have confounded, are as far distant, as a state of peace, good will, mutual assistance and preservation, and a state of enmity, malice, violence and mutual destruction, are one from another. 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 [emphasis added].
John Locke, The Second Treatise of Government, 1690.