Question: When can a country intercept a vessel?

Background: Answer: If it clearly intends to illegally enter another country’s waters – as soon as it leaves its port of origin.

The 'Mavi Marmara' 311 (R) (photo credit: Reuters/Emrah Dalkaya)
The 'Mavi Marmara' 311 (R)
(photo credit: Reuters/Emrah Dalkaya)
A maritime blockade is a tool countries can employ during an armed conflict to stop the passage of weapons and other goods to a country or entity. Israel has imposed one on Gaza since January 3, 2009, during Operation Cast Lead.
One of the main arguments raised against the boarding of the six ships associated with May 2010’s flotilla was that the action reportedly took place 70 nautical miles (130 kilometers) from the coast, inside international waters.
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Immediately following the raid, in which nine activists on the Mavi Marmara were killed, Turkey drafted a resolution at the UN Security Council that described the boarding of the ship as a violation of international maritime law.
Under the San Remo Manual, a 1994 legally recognized but nonbinding document from the International Institute of Humanitarian Law dealing with blockades and armed conflicts – widely held to be the international standard for blockades – countries can intercept a vessel that is trying to breach the blockade as soon as it leaves its port of origin if the vessel is clear in its purpose to illegally enter waters, a legal adviser to the Foreign Ministry who is an expert in maritime law explained.

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The official added that blockades are always imposed in international waters, and ships trying to breach the blockade are intercepted even further out, to give the military enough time to intercept it before it reaches land.
There are a number of conditions that a blockade must satisfy to be a “legal blockade” under the San Remo Manual. Countries imposing a blockade must give prior notice, and inform shipping companies and other countries where and when the blockade is being enacted, with ample time for vessels to alter their routes. Countries must also enforce the blockade in a nondiscriminatory manner, meaning that no ship, peaceful, commercial or otherwise, can enter the area of the blockade for any reason.
Additionally, the blockade must not block access to neutral ports or neutral waters – in this case Egyptian ports, for example, or major shipping lanes between countries not involved in the conflict. Finally, countries cannot use a blockade to deny the civilian population items essential to their survival.
Flotilla organizers seized on this last point to explain their reasons for trying to breach the blockade.
Blockades have been used throughout history, including in the Vietnam War, World Wars I and II, and recently in Sri Lanka as the country was struggling to control the Tamil Tigers terrorist group. Since the end of March, there has been an air and naval blockade imposed by NATO against Libya.
Gaza has no territorial waters, as it is not a country. Israel has sovereignty over the area up to 12 nautical miles from its coast. The Gaza blockade extends 20 nautical miles from the coast, or approximately 37 km. A nautical mile, the standard measurement of distance on the ocean, is 1.852 km.