Under both customary international law and the Vienna Convention on Diplomatic and Consular Relations of 1963 (of which IIRC the U.S. is not a member but generally abides by its provisions), property of foreign missions (whether embassies or something less formal) or the facilities of non-governmental organizations like the UN or NATO is not "foreign soil" as the movies would have you believe -- it is and always remains the property of the host country. However, the host country makes certain promises when granting permission for a foreign on NGO entity to establish a mission; in particular, it promises that it will hold the facilities inviolate and will not enter or otherwise harass them or the personnel without that government's permission.
We could witdraw from the UN tomorrow and require them to get their asses out of our country IF we had the cajones to do it!
A special headquarters agreement, signed by Secretary-General Trygve Lie and US secretary of state George C. Marshall at Lake Success on 26 June 1947, has been in force since 21 November 1947.
It defines the 18 acres of land in New York City located between 42nd and 48th Streets and First Avenue and the Franklin D. Roosevelt Drive as the Headquarters District of the United Nations.
Subsequently, by supplemental agreements between the UN and the US, additional office space located in buildings in the vicinity has been included in the Headquarters District.
The Headquarters District is "under the control and authority of the United Nations as provided in this agreement."
It is the seat of the UN, and the agreement stipulates that the district "shall be inviolable."
Federal, state, and local personnel on official duty may enter it only with the consent of the Secretary-General.
The UN may make regulations for the area.
US federal, state, and local law, insofar as it is inconsistent with UN regulations, does not apply here; otherwise, the US courts would have jurisdiction over actions and transactions taking place in the Headquarters District.
Read more:
http://www.nationsencyclopedia.com/United-Nations/United-Nations-Headquarters-HEADQUARTERS-AGREEMENT-BETWEEN-THE-UN-AND-THE-US.html#ixzz4U8zx62XF