A subpoena loses its coercive effect the instant it leaves its own jurisdiction — even within the United States.  When that same subpoena leaves the country, the involvement of a foreign court is necessary to compel testimony or the production of documents. Foreign courts take a very dim view of American-style discovery, so requests for judicial assistance must be carefully crafted, lest they be rejected entirely. Viking Advocates can advise counsel on the practicality of such a request, draft the request itself, and recruit foreign counsel to shepherd the request through the foreign court.  See here and here for more.