A long-standing common-law policy holds that anyone may lawfully use force to repel or arrest a criminal threatening property, and a fortiori that force may be used to defend one’s own property. But there are limits to these powers. In cases where some amount of violence is justified but excessive force is used, some common-law jurisdictions will deny any defence to murder. Killing through excessive force is neither justified nor excused. Other jurisdictions will allow a partial defence, excusing from the fullest penalty and reducing the offence to manslaughter or unlawful homicide, on the ground that the actor should be punished for a grievous miscalculation. By analyzing the relationships between persons and property and investigating the historical development of self-help doctrines, the principles defining excessive force here may be reduced to four. First, property cannot easily be conceived as a value worth protecting by force where there is no accompanying threat to the person. Second, the law may nonetheless see a presumptive identity between an owner’s person and his property which is external to his person, so that violent defence of property becomes justified even without a threat to bodily or personal safety. Third, intentionality and purpose play an important role in attributing legal blame for excessive force in protecting property, and the test of allowable quantum of force is not objective. Finally, the legal and ethical issues are transformed when we move from individual to collective property. Judgments concerning state and international legality regarding excessive force may derive only limited guidance from the contours of criminal law and private law. Yet, much of international conflict today is carried out by private actors who do not wear uniforms or wrap themselves in the shield of state immunity, and the domestic legal dimension thus begins to have purchase on issues of excessive force in war and communal conflict.