The terms of the Hague Convention, which came into force in 1910, outlined the rights and responsibilities of a neutral power in 25 Articles which can be read in full here.
Written at a time when land warfare was the rule, it is now considered that Article 2 of the Hague Convention implies that the neutral states must not allow a belligerent nation, either the passage or over-flight by military vehicles or aircraft. This prohibition applies not only for fight jets but also for transport of troops and provisions.
Art. 1. The territory of neutral Powers is inviolable.
Art. 2. Belligerents are forbidden to move troops or convoys of either munitions of war or supplies across the territory of a neutral Power.
Art. 3. Belligerents are likewise forbidden to:
(a) Erect on the territory of a neutral Power a wireless telegraphy station or other apparatus for the purpose of communicating with belligerent forces on land or sea;
(b) Use any installation of this kind established by them before the war on the territory of a neutral Power for purely military purposes, and which has not been opened for the service of public messages.
Art. 4. Corps of combatants cannot be formed nor recruiting agencies opened on the territory of a neutral Power to assist the belligerents.
Art. 10. The fact of a neutral Power resisting, even by force, attempts to violate its neutrality cannot be regarded as a hostile act.
Art. 11. A neutral Power which receives on its territory troops belonging to the belligerent armies shall intern them, as far as possible, at a distance from the theatre of war.
Art. 12. In the absence of a special convention to the contrary, the neutral Power shall supply the interned with the food, clothing, and relief required by humanity.
Art. 13. A neutral Power which receives escaped prisoners of war shall leave them at liberty. If it allows them to remain in its territory it may assign them a place of residence. The same rule applies to prisoners of war brought by troops taking refuge in the territory of a neutral Power.
Art. 14. A neutral Power may authorize the passage over its territory of the sick and wounded belonging to the belligerent armies, on condition that the trains bringing them shall carry neither personnel nor war material.
Art. 15. The Geneva Convention applies to sick and wounded interned in neutral territory.
Art. 7. A neutral Power is not called upon to prevent the export or transport, on behalf of one or other of the belligerents, of arms, munitions of war, or, in general, of anything which can be of use to an army or a fleet.
Art. 8. A neutral Power is not called upon to forbid or restrict the use on behalf of the belligerents of telegraph or telephone cables or of wireless telegraphy apparatus belonging to it or to companies or private individuals.
A list of thirteen countries or states recognised as neutral may be read here.