The new subpoena threatened by the PSNI against the Belfast Project at Boston College will have to be served via the 1994 Mutual Legal Assistance Treaty (MLAT) which gives either of the parties, the US government or the UK administration, the power to stop any threatened action because of “rights or obligations” under other international agreements.
Article 18, section 1 of the MLAT has this to say:

The Parties, or Central Authorities, shall consult promptly, at the request of either, concerning the implementation of this Treaty either generally or in relation to a particular case. Such consultation may in particular take place if, in the opinion of either Party or Central Authority, the expenses or other resources required for the implementation of this Treaty are of an extraordinary nature, or if either Party has rights or obligations under another bilateral or multilateral agreement relating to the subject matter of this Treaty.

This means that Eric Holder, the US Attorney-General has the right to ask his opposite number in Britain, Home Secretary, Theresa May to halt the subpoena on the grounds that the PSNI move would harm US interests in relation to the 1998 Good Friday Agreement.

Holder would be entitled to say that since the Good Friday Agreement is in large measure the creation of the US government – Presidents Clinton and George W Bush played a major role in both bringing it into being and implementing it – then US foreign policy interests would be adversely affected by the plunder of the oral history archive.

For example, the PSNI action is so unprecedented and extreme that it risks undermining one of the pillars of the new political dispensation in the North, Nationalist support for policing, a development that would be of concern in Washington.

Whether Holder does this is entirely dependent on the political pressure applied on the Obama White House, especially by Irish-America. The police assault on Boston College, and through that on the Good Friday Agreement and its architects, is so unprecedented that some sort of reaction from Irish-America seems likely.

The PSNI action against the Boston archive is, for example, in stark contrast to the force’s inactivity in relation to proven and deadly instances of collusion between named and known members of the security forces, including the old Special Branch, and Loyalist paramilitaries, collusion that was likely known about and approved by the British intelligence agency, MI5.

While the PSNI has pursued the Boston archive with vigour it has done next to nothing to bring security force members to book for involvement in murder. For example a 2007 report by police ombudman Nuala O’Loan which exposed RUC Special Branch handlers who had turned a blind eye to their agent’s involvement in a string of killings has gathered dust since. The full report can be read here.

The most generous interpretation of the latest PSNI move is “the lunatics are running the asylum” theory, which says that either the senior management of the force does not know or care what the consequences of this action are. The other, more malign interpretation is that the leadership of the PSNI have a very clear idea of the potential damage and, under cover of pursuing historical crime, are motivated by a desire for revenge against those seen as responsible for changes in policing such as the disbandment of the Special Branch.