While nuclear-armed states and their allies claim that existing nuclear sharing practices are permissible under the provisions of the NPT, these arrangements undermine the object and purpose of the treaty- to which all these states are party. The NPT  was created to stop the spread of nuclear weapons and explicitly prohibits the transfer of control, or accepting control, over the nuclear weapons of a nuclear weapons state, under articles 1 and 2.  In spite of this, the US has maintained that the practice of deploying weapons to another country and preparing to transfer control is not explicitly forbidden under the agreement.

However, the UN Treaty on the Prohibition of Nuclear Weapons (TPNW) is explicit in prohibiting countries from hosting any  nuclear weapons. As the TPNW went into legal effect in 2021, nuclear sharing is now illegal under international law. The TPNW now forms the basis for a strong norm against hosting or stationing, and all responsible states should join this treaty.

If more states were to engage in nuclear sharing, it raises many worrying questions around the legal limits that undermine the global disarmament architecture and international security:  Can all nuclear-armed states lawfully deploy their nuclear weapons on the territories of other states? Can all 185 non-nuclear-weapon states, as defined by the NPT, accept the deployment of such weapons on their territories? Can any country train military personnel to accept control of the weapons?

Related questions:

What is nuclear sharing and who is involved?
What are the risks of nuclear sharing?