What is a Blockade?

The term “blockade” is often used colloquially to refer to various operations or interdictions that restrict maritime traffic. Legally, it has a specific meaning. A blockade is designed to prevent vessels and aircraft of all states—enemy and neutral—from entering or exiting specific ports, airfields, or coastal areas under the control of an enemy state. The purpose of a blockade is to deprive an adversary of the supplies it needs to keep fighting.

A blockade is a belligerent action—or in non-legal terms, an “act of war.” If a ship or aircraft of a neutral state attempts to breach the blockade, it may be subject to capture.

To be legal, a blockade must be established by proper legal authority. In the United States, only the President or the Secretary of Defense can declare a blockade. While the President’s choice to declare a blockade on social media was novel, it was legal. The notice of a blockade may be made in any form so long as it is effective.

For a blockade to be binding under international law, it must also be effective. This means that the blockade must be maintained by forces that are sufficient to make it dangerous to enter or exit the blockaded area. It must also be applied impartially to vessels of all states, although special entry and exit may be allowed in certain circumstances for neutral government and vessels and for humanitarian reasons.

Blockades are often confused with maritime quarantines. A maritime quarantine is a peacetime military action that is a measured response to an acute threat or crisis with the goal of de-escalation and return to stability. By contrast, a blockade is an act of war against an identified belligerent party with the ultimate goal of degrading and defeating the enemy. The United States has invoked a maritime quarantine only once, by President John F. Kennedy in 1962 to interdict the flow of Soviet missiles and weapons into Cuba.

What is Being Blockaded?

U.S. Central Command’s press release announcing that it would implement the blockade covers only maritime traffic entering and exiting Iranian ports and explicitly preserves freedom of navigation for vessels transiting the Strait to and from non-Iranian ports. Blocking enemy ports is a typical form of naval blockade between belligerent parties.

Won’t this Hurt Other States Besides Iran? What About Freedom of Navigation?

The United States’s aim is to restore freedom of navigation. International law regarding blockades exists to ensure that freedom of navigation continues for neutral parties. Neutral states retain the right to engage in neutral commerce that does not involve trade or communications originating in or destined for the blockaded area.

The blockade is designed to apply pressure on Tehran while allowing trade and navigation to continue. Legally, a blockade cannot block neutral nations’ access to their own ports and coastlines, and cannot prevent trade that does not originate from or go to the blockaded area. U.S. Central Command has explicitly stated that U.S. forces “will not impede freedom of navigation for vessels transiting the Strait of Hormuz to and from non-Iranian ports.”

Blockades must also comply with the law of armed conflict, which includes not targeting innocent civilians. To protect civilians, international law requires the blockading party to notify all affected states before the blockade takes effect. The notification must specify, at minimum, the start date, geographic scope, and grace period for neutral vessels to clear the area. The length of the grace period is determined by the belligerent based on what it considers reasonable under the circumstances. Accordingly, U.S. Central Command’s initial press release regarding the blockade stated that additional information would be provided to commercial mariners through a formal Notice to Mariners, and advised all mariners to monitor bridge-to-bridge channel 16 when operating in the Gulf of Oman and Strait of Hormuz approaches.

The United Kingdom Maritime Trade Operations, the functional clearinghouse for notices to mariners in the Persian Gulf for the past 25 years, issued a more detailed Notice to Mariners on April 13. The notice confirms that the blockade “is not reported” to impede transit passage through the Strait to or from non-Iranian destinations.

What Does China Have to Do With All of This?

Because of U.S. sanctions on Iran, China is the sole large-scale buyer of Iranian oil. Iranian oil is typically shipped by an illicit shadow fleet, ships that obscure their ownership, flag, or cargo to evade sanctions and oversight. Under international law, every ship must bear the flag of a state responsible for its actions. Shadow fleet vessels operate under what is known as “flags of convenience,” which means purchasing a flag from a state that usually has no ties to the actual state. For example, an Iranian tanker might purchase a flag from Liberia, and then use that vessel to transport sanctioned oil to China.

The U.S. blockade indirectly targets China’s relationship with Iran. A successful blockade will squeeze Iran’s oil revenues and keep Iranian oil out of China’s hands. The blockade also allows U.S. forces to more closely scrutinize the shadow fleet activities that have helped keep the regime afloat.

Will This End the Cease-Fire?

According to Iran, the cease-fire was already broken. Iran and Pakistan, which helped broker the cease-fire, claim that the cease-fire extended to Israel’s operations in Lebanon. The U.S. and Israel disagree. Some Iranian strikes reportedly occurred in Gulf States after the cease-fire began. Legally, the blockade is a belligerent act, and Iran’s paramilitary Islamic Revolutionary Guard Corps has already stated that a blockade would violate the cease-fire.

The U.S. Navy in the ironic position of enforcing a blockade to restore freedom of navigation. Before the blockade, only 23 ships had passed through the Strait since the cease-fire began. Before the conflict, the Strait saw 138 vessels pass per day. Iran has weaponized the Strait and is demanding sovereignty over it as a condition of ending the war. The U.S. blockade is designed to end Iran’s chokehold on global trade in the short term, and to show Iran that the world will never allow Iranian sovereignty over the Strait in the long term.