Under international and US law, blockades are acts of war, variously defined as:
-- surrounding a nation or objective with hostile forces;
-- measures to isolate an enemy;
-- encirclement and besieging;
-- preventing the passage in or out of supplies, military forces, or aid in time of or as an act of war; and
-- an act of naval warfare to block access to an enemy's coastline and deny entry to all vessels and aircraft.
Law Professor Francis Boyle calls blockades:
"belligerent measures taken by a nation (to) prevent passage of vessels or aircraft to and from another country. Customary international law recognizes blockades as an act of war because of the belligerent use of force even against third party nations in enforcing the blockade. Blockades as acts of war have been recognized as such in the Declaration of Paris of 1856 and the Declaration of London of 1909 that delineate the international rules of warfare."