Hugo Grotius (15831645). The Rights of War and Peace. 1901.
Chapter XVII: Respecting Those Who are Neutral in War
Nothing to be taken belonging to neutrals, but under circumstances of extreme necessity, and with an intention to pay the full price of itConduct of neutral powers towards belligerents.
I. IT may appear superfluous to speak of neutral powers, against whom no rights of war can exist. But as war, under the plea of necessity, occasions many aggressions to be committed against them, especially when bordering upon the seat of its operations, it may be necessary briefly to repeat a former assertion, that nothing short of extreme exigency can give one power a right over what belongs to another no way involved in the war. The case too is equally clear that no emergency can justify any one in taking and applying to his own use what the owner stands in equal need of himself. But even where the emergency can be plainly proved, nothing can justify us in taking or applying the property of another to our use, beyond the IMMEDIATE DEMANDS OF THAT emergency. Where the CUSTODY of a thing, by securing it, is sufficient for the purpose, the USE and CONSUMPTION of it is absolutely unlawful. If the USE of it is necessary, it must not be ABUSED: and if the entire ABUSE of it be requisite, the full value should be paid.
II. Again, according to what was said in a preceding part of this book, it is the duty of those, who profess neutrality in a war to do nothing towards increasing the strength of a party maintaining an unjust cause, nor to impede the measures of a power engaged in a just and righteous cause. But in doubtful cases, they ought to shew themselves impartial to both sides, and to give no succour to besieged places, but should allow the troops of each to march through the country, and to purchase forage, and other supplies. The Corcyraeans, in Thucydides, say that if the Athenians intend to remain neuter, they ought either to prohibit the Corinthians from enlisting men in the territory of Attica, or to give THEM the same privilege. The Romans objected to the conduct of Philip king of Macedon, charging him with a double violation of treaties, both by injuring the allies of the Roman people, and assisting the enemy with supplies of men and money.