USS Lassen (DDG-82) is underway in the Philippine Sea in 2013. US Navy Photo
Much has been made about the recent transit of the USS Lassen (DDG-82) within 12 nautical miles of Subi Reef in the South China Sea. To review, that is because China has built an artificial island on top of Subi Reef, claims it as sovereign territory, and claims territorial rights to the seas around the reef.
It is important to note that Subi Reef is naturally a “low-tide elevation”; that is, it is only exposed to the atmosphere at low tide. In its natural state, before China built an artificial island on top of it, the reef was entirely submerged at high tide. Under international law, natural low-tide elevations do not have territorial waters associated with them. Building an artificial structure, including an island, on top of a low-tide elevation does not grant it additional territorial attributes: it is still a low-tide elevation under international law.
The United States has a longstanding program of challenging maritime claims that it believes are not consistent with international law, and conducts freedom of navigation (FON) operations to challenge those claims. The Center for Strategic and International Studies (CSIS) Asia Maritime Transparency Initiative (AMTI) maintains a good primer on FON operations, and there are a number of good articles on the subject of Subi Reef and the Lassen transit that discuss those issues.
Of concern, however, is the characterization of this transit as being one of “innocent passage.” Innocent passage is the right of any ship to transit the territorial waters of another nation, as long as it is “innocent”. For warships, that includes operating in a non-confrontational manner, such as not exercising or practicing weapons, conducting surveillance, launching or recovering aircraft, and so forth.

Read more: http://news.usni.org/2015/11/04/opinion-uss-lassens-transit-of-subi-reef-was-not-so-innocent