"The Proliferation Security Initiative (PSI) is a kind of security regime which was established at President Bush's suggestion in May 2003. Its purpose is to intercept vehicles, ships, and airplanes in order to protect against the proliferation of weapons of mass destruction (WMD). Seventeen countries including the United States have currently joined in the PSI, and over sixty countries in the world support this initiative. PSI activities have been mainly focused on the interception of ships, which has had a practical effect since 2003. The reason is that the proliferation of WMD is generally accomplished by transportation using ships. However, these activities have posed some problems: 1) The innocent passage or freedom of navigation in the United Nations Convention on the Law of the Sea (UNCLOS) might be undermined. 2) China and Russia as major powers are not active participants in these activities. 3) The object of interception is willfully categorized as aiming at certain rogue states. 4) Identification and interdiction are usually difficult as long as the shipping materials aredual-use items which are generally comprised of WMD-related materials. Although these problems are not easily resolved, the PSI has helped to settle them thanks to the expanding trend toward international consensus on the non-proliferation of WMD. In particular, United Nations Security Council Resolution (UNSC) 1540, which was agreed upon in April 2004, requests member states to take effective domestic, border, export, and transportation controls focused on WMD-related materials. Although that resolution did not provisionally address interdiction through checks and inspections, it is recognized to have legitimacy in terms of international law since it is interpreted as a pro-active activity to implement the resolution."
Naval Postgraduate School, Center for Contemporary Conflict: http://www.ccc.nps.navy.mil
Strategic Insights (August 2006,) v.5 no.7