The principles of modern international law recognize the doctrine of blockade as a legitimate option between states in a declared state of war, giving them the right to apply naval power to stop all seaborne commerce with the enemy, including that carried in neutral ships.
McNulty, James F.
"School of Naval Command and Staff: Blockade: Evolution and Expectation,"
Naval War College Review: Vol. 19
, Article 6.
Available at: http://digital-commons.usnwc.edu/nwc-review/vol19/iss6/6