The legal positions (definitely plural) are described in Pakistan's Sovereignty and the Killing of Osama Bin Laden, by Ashley S. Deeks, referenced in my post #21 above.

My own analysis starts with a first proposition that India, Pstan, Astan and the US are all 1949 Geneva states; have not accepted 1977 AP I and AP II in toto; and accept the Hague regulations. My second proposition is that a 1949 Geneva nation may engage in an armed conflict with a non-state actor (a "Power" in the conflict, which has an option to accept and apply 1949 Geneva under Common Article 2 and generally will come under Common Article 3).

The third proposition is that the Hague regulations impose duties on a neutral state, Convention (V) respecting the Rights and Duties of Neutral Powers and Persons in Case of War on Land. The Hague, 18 October 1907:

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 forthe 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. 5. A neutral Power must not allow any of the acts referred to in Articles 2 to 4 to occur on its territory.
From that obligation of the neutral Power, follows the conclusion found in our FM 27-10, The Law of Land Warfare:

520. Effect of Failure to Prevent Violation of Neutrality by Belligerent Troops

Should the neutral State be unable, or fail for any reason, to prevent violations of its neutrality by the troops of one belligerent entering or passing through its territory, the other belligerent may be justified in attacking the enemy forces on this territory.
Of course, simply because you have a hunting license does not mean you should kill everything in the forest.

The real question is not legality, but the likely response - diplomatic protest vice nuclear warhead.

Regards

Mike