De iure belli et pacis, Book 2, Chapter 25
VI. The last and most extensive motive is the common tie of one COMMON NATURE, which alone is sufficient to oblige men to assist each other. VII. It is a question, whether one man is bound to protect another, or one people another people from injury and aggression. Plato thinks that the individual or state not defending another from intended violence is deserving of punishment. A case for which provision was made by the laws of the Egyptians. But in the first place it is certain that no one is bound to give assistance or protection, when it will be attended with evident danger.
For a man's own life and property, and a state's own existence and preservation are either to the individual, or the state, objects of greater value and prior consideration than the welfare and security of other individuals or states.
Nor will states or individuals be bound to risk their own safety, even when the aggrieved or oppressed party cannot be relieved but by the destruction of the invader or oppressor. For under some circumstances it is impossible successfully to oppose cruelty and oppression, the punishment of which must be left to the eternal judge of mankind.
See also Pufendorf, The Whole Duty of Man, p. 242 (Hunter and Saunders translation)