As readers of Just Security are no doubt aware, Iran’s shoot down of an unmanned U.S. Navy RQ-4A Triton surveillance aircraft in the Persian Gulf has elevated tensions between the United States and Iran. What has been largely missing from the public discussion is that even if the drone flew only across “international airspace,” significant legal questions arise as to whether its activities violated the rules for transit and what legal recourse Iran then had to respond.

This was reportedly not the first such action by Tehran in recent days. Iran reportedly fired on and missed a U.S. unmanned aircraft the week prior. The U.S. has already launched a cyber operation against Iran.  In addition, Iran allegedly attacked non-U.S. commercial shipping in the region last week — although details about these attacks remain murky as well.  Secretary of State Mike Pompeo has said that these earlier attacks were intended to deny transit through the Strait of Hormuz, a critical international strait between Oman and Iran where 20 per cent of the world’s oil transits. Plans for a non-cyber retaliatory strike are on hold, but the situation remains tense.

Iranian and United States Competing Facts

 Iran claims that the Triton was operating in Iranian airspace near the Strait of Hormuz. It asserts that the use of force against the Triton was consistent with the exercise of the inherent right of self-defense in Article 51 of the U.N. Charter. As Ashley Deeks and Scott Anderson have astutely pointed out, a state is entitled to control access to national airspace above its territorial sea. But the Iranian use of force is complicated by two additional factors: (1) the unique navigational rights afforded to foreign ships and aircraft operating in the Strait of Hormuz; and (2) Iran’s “straight baseline” territorial claim in the vicinity of the Strait of Hormuz that is not recognized by the United States.

In contrast, the United States has provided evidence that the drone was 21 miles from the Iranian coastline, outside Iranian territorial airspace. In addition, aircraft — to include unmanned aircraft such as the Navy Triton— enjoy special transit passage rights through straits connecting one part of the high sea or exclusive economic zone with another, such as the Strait of Hormuz. While the United States has not specifically stated that the Triton was engaged in transit passage through the Strait of Hormuz, it has nevertheless maintained that the Triton was operating over the Strait of Hormuz. Transit passage is subject to a few conditions delineated by customary international law and the UN Convention on the Law of the Sea (UNCLOS). Thus, understanding whether the U.S. Navy Triton was operating lawfully, and whether Iran had grounds to shoot it down, is not a simple matter of determining whether the Triton was inside or outside Iranian airspace. The analysis must take into account this special navigational regime as well as the overall legitimacy of Iran’s contested territorial claims.

Transit Passage Rights and the Strait of Hormuz

The U.S. Navy has consistently stated that transit passage rights apply to the Strait of Hormuz as it is a strait connecting one part of the high sea or exclusive economic zone with another — akin to the Strait of Malacca in Malaysia and the Strait of Gibraltar that connects the Atlantic Ocean with the Mediterranean Sea. While the United States is not a party to UNCLOS, it has long maintained that the treaty’s transit passage rights are reflective of customary international law. UNCLOS Articles 38 and 39 outline the right and duties of transit passage afforded to foreign vessels (here, the United States) and the coastal state (Iran). It is generally understood that these UNCLOS provisions apply to “all ships and aircraft”— to include the unmanned Triton. As a state bordering the Strait of Hormuz, Iran may not hamper transit passage and may not suspend transit passage. The alleged Iranian mining of the Japanese and Norwegian commercial vessels in the vicinity of the Strait of Hormuz appear to violate this prohibition.

As a general matter, a military aircraft ­may exercise the right of unimpeded transit passage as it proceeds through the Strait of Hormuz provided that it complies with certain provisions. First, the transit must be continuous and expeditious and “in the normal modes of operation” for such passage. Second, the aircraft must proceed without delay and must refrain from any use of force. Third, “research or survey activities” are prohibited under Article 40 of UNCLOS while conducting transit passage. Finally, the aircraft must refrain from any activities other than those incident to the normal modes of continuous and expeditious transit. Finally, while transit rights are afforded in the Strait of Hormuz, that “does not otherwise affect the legal status of the waters [and airspace] forming such straits. . .”  Outside the Strait of Hormuz and Iranian airspace, normal high seas freedoms for vessels and aircraft apply.

In its letter to the UN Security Council, Iran states that the Triton was outside the Strait of Hormuz while returning “towards the Western parts of the region.” Iran has maintained that it made “repeated radio warnings” but the United States has yet to comment on this claim, so we do not know for certain how this communication unfolded. Of note, UNCLOS requires that aircraft must monitor international radio frequencies while in transit passage. In contrast, the United States has stated that the Triton was 21 miles from the nearest point in Iran and has provided some credible evidence to bolster this assertion (although it was called into question in subsequent reporting).

Assuming the aircraft was operating within the Strait of Hormuz where transit passage rights apply, the question becomes whether the U.S. Triton complied with the duties associated with transit passage. The law is unclear on what, exactly, encompasses a “normal mode of operation” for an unmanned aircraft. Iran could credibly argue that surveillance activities are analogous to, if not more severe than, the prohibited “research or survey” activities and the Triton was not operating in a continuous and expeditious manner. The Triton is not a stealth aircraft per se, but it was allegedly operating in stealth mode and not reporting its location. Iran could then take measures if the aircraft is in violation of transit passage rights provided that Iran could exercise sovereignty over the airspace.

But UNCLOS is a bit vague on the specific measures a coastal state may take if transit passage rights are violated, as is international law more generally. UNCLOS speaks generally of the requirement to resolve disputes by “peaceful means.” Regardless, even if forcible measures are in principle permitted, the Iranian kinetic response is difficult to justify and appear excessive, despite the unmanned nature of the Triton. This is particularly true when taking into account the underlying transit passage regime, international law’s focus on settling disputes by peaceful means, and the Triton’s very actions — it was not operating in any sort of attack profile that could be reasonably perceived as a hostile act, as surveillance alone should not be considered hostile action.

The Ongoing Impasse on the Straits of Hormuz

This analysis would differ had the Triton entered uncontested Iranian airspace and ignored warnings, as Iran claims. Outside this particular incident, for decades the United States and Iran have disagreed about the legal regime governing Strait of Hormuz transit. The Strait of Hormuz is central to global trade and national security — 20 percent of the world’s oil travels via this key navigational waterway and foreign warships routinely make the journey. Further complicating matters, neither the United States nor Iran are parties to the UN Convention on the Law of the Sea. Iran has argued that the generous transit passage rights codified in UNCLOS do not apply to non-parties such as the United States and has consistently asserted a “straight baseline” maritime claim.  Accordingly, Iran has historically taken a narrow view of the United States’ navigational rights in the Strait of Hormuz.

Not surprisingly, the United States disagrees about both of these assertions, labeling Iran’s claims as excessive. The United States has historically treated the major navigational provisions in UNCLOS, such as transit passage, as reflective of customary international law and has abided by and accepted these provisions throughout the world.  Due to this long-standing impasse, Professor James Kraska of the Naval War College has perceptively labeled the underlying legal uncertainty governing the navigational regime in the Strait of Hormuz a “legal vortex.”

So where do we go from here? The Triton shoot down has brought this historical uncertainty regarding the Strait of Hormuz to a head. And the stakes could not be higher. The United States came within minutes of launching a major kinetic attack against Iran in response to the attack and reports indicate that a cyberattack against Iran took place.

What is clear is that the U.S. military will continue to operate in the Strait of Hormuz and the surrounding area and Iran will aggressively defend its airspace against any real or perceived entry. And there is no evidence to suggest that it will back down from its straight baseline territorial claims over its adjacent sea and airspace. We should stop the name-calling and focus on ways to defuse this crisis to avoid miscalculation and miscommunication — we simply can’t afford to walk blindly into a disaster.

Photo: Federica Grassi/ Getty Images