Locke starts off by defining war as a state of "enmity and destruction" brought about by one person's pre-meditated attempts upon another's life. The law of self-preservation, integral to the law of nature, dictates that a person may kill another person in self-defense. This definition rests upon the presumption that any aggression by one person against another constitutes a challenge to that person's freedom. By this reasoning, one can justifiably kill a thief since an attack on one's property represents a threat to one's liberty.
Locke then outlines the differences between the state of nature and the state of war, noting that the two are NOT the same. The state of nature involves people living together, governed by reason, without a common superior, whereas the state of war occurs when people make designs of force upon other people, without a common authority. In this case, the attacked party has a right to war. Want of a common judge or authority is the defining characteristic of the state of nature; force without right is adequate basis for the state of war.
The difference between war in Society and war in Nature depends on when they conclude. In Society, war ends when the "actual force is over," because both parties can then resort to the common authorities for arbitration of past wrongs. In Nature, war does not end until the aggressive party offers peace and reparations for the damage done; until then, the innocent party has a right to try to destroy the aggressor. Locke notes that in the presence of a common authority that fails to act justly, the only possible state is a state of war, because the arbitrating power in place to stop war is itself in violation of the laws of nature and justice. Locke ends the chapter by noting that one of the major reasons people enter into society is to avoid the state of war, for the presence of a supreme power limits the necessity for war and increases stability and security.
Locke starts Chapter 4 by defining natural liberty as a person's right to be ruled solely by the laws of nature, and social liberty as the right to be under no legislative power other than that founded by the consent of the commonwealth, functioning for the commonwealth's benefit.
Locke bases his ideas about slavery on the idea that freedom from arbitrary, absolute power is so fundamental that, even if one sought to, one could not relinquish it; it is therefore impossible for one to enlist into slavery voluntarily. The only possible state of slavery is the extension of the state of war, between a lawful conqueror and a captive, when the captive has been forced into obedience. Locke notes that even in Exodus, the Jews did not sell themselves into slavery, but simply into drudgery, for their masters did not have full power over their lives, and therefore, did not have full control over their liberty.
We should note that Locke's use of the term "war" really means "conflict," since he addresses clashes between individuals rather than nations. In the state of nature, the absence of authority requires individuals to protect themselves. In society, victims can appeal to a common authority for the resolution of disputes, when possible (there are times that this is impossible, as in Locke's justification for killing the thief). Locke's definition of what constitutes, justifies, and ends a state of war continues his explication of the natural foundation of government. We can see more and more how fundamentally all of Locke's ideas rest on the right to personal liberty, and in the next section we will see that he directly equates that libertywith property, making property the Treatise's most important subject.