The history of balloons flying into the sovereign airspace of other states is not new. Hundreds of balloons released by the US Air Force as part of Project Genetrix in 1956 overflew parts of Eastern Europe and the then Soviet Union, drawing diplomatic protests from various governments. In 1995, two American balloonists taking part in a race were shot down by Belarus after their balloon strayed too close to restricted airspace. The incursion of military aircraft into Pakistan/ India airspace frequently becomes hot topic raising tension between the two nuclear states. Thus, in law and based on the practice of states, overflight is strictly prohibited, and states will, if necessary, shoot down the objects that have infringed their sovereignty and sovereign airspace.

Apart from this, there are unfortunate incidents of the shooting down of civil aircraft carrying passengers onboard, which have led to the loss of many innocent civilian lives. As a result, Chicago Convention explicitly prohibits "resorting to the use of weapons against civil aircraft in flight". Conversely, states have an obligation to ensure that their civil aircraft, whether piloted or not, are not used in violation of international law and do not infringe the sovereign rights of other states.

The balloon reportedly entered United States (US) airspace over Alaska on 28 January and 2 February 2023, respectively. The balloon that entered the Canadian and US airspace appears to have originated from central China and flew through Alaska, British Columbia, and then over Montana, a vital part of the US nuclear establishment. After over 10 days of floating, on 4 February 2023, the balloon was eventually shot down by the US Air Force. On 3 February, China spoke out that the 'balloon' is, in fact, "a civilian airship used for research, mainly meteorological, purposes". According to the provision of International Civil Aviation Organization (ICAO), "unmanned free balloon", is defined as a "non-power-driven, unmanned, lighter-than-air aircraft in free flight". In terms of the right-of-way of aircraft though, it differs and depends on how maneuverable the aircraft, with airships needing to give way to balloons as the balloon moves with the help of wind.

The Pentagon USA and Canada claimed that the size of the balloon and its ability to change course were nothing short of an airship and a violation of sovereign airspace and international law. Though, Beijing admitted the entry of the Chinese airship saying "no intention to violate and has never violated the territory or airspace of any sovereign country"; in turn, it regretted the unintended entry of the airship and clarified that the entry into the US airspace to be "due to force majeure and was totally unexpected and accidental".  

In fact, article 3bis were added to the Chicago Convention based on the recognition that states are entitled to take appropriate measures to safeguard their aerial sovereignty and rights, and obligations under the Charter of the United Nations, while other states must not violate other states' airspace nor use "civil aviation for purposes inconsistent with the aims of the Convention". It is arguably only relevant with respect to commercial passenger airliners but the Chinese object which overflew Canada and the US was neither an airliner nor was there any person on board.

The 1944 Chicago Convention clearly stipulates that "every State has complete and exclusive sovereignty over the airspace above its territory a "principle firmly established tenets of customary international law". The Chicago Convention applies to civil (as opposed to state) aircraft, whether it is an airship, as per the Chinese government, or a balloon, per the USA, Canada and other overflown states; both categories of aerial objects fall under the definition of aircraft, and are therefore subject to international air law.

Presuming the object is a balloon, according to ICAO standards, an unmanned balloon cannot be operated without an appropriate authorisation from the state from which the launch is made. Thus, the balloon's entry into another state's airspace is a violation of international law.

Evidence suggests that China did not obtain authorisation from the overflown states or make the necessary notification to air traffic services about the launch and the trajectory of the balloon. Additionally, China did not fulfil its obligation to terminate the operation of the balloon prior to the unauthorised entry into the airspace over another state's territory.

If the object in question were categorised as a military aircraft, as many states and experts believe it is, there is even less of a right to overfly the territory of another state according to the Chicago Convention. Even assuming the aerial object is a civilian aircraft, as China claims, it has been argued that the legal characterisation of an aircraft depends on its actual 'use'. Thus, an object that on the surface is a civil aircraft used for military purposes may be deemed a military aircraft.

Conversely, for force majeure to be successfully pleaded, there must be an "irresistible force or [...] an unforeseen event" beyond the control of the state, which makes it "materially impossible in the circumstances to perform the obligation".  Admitting that the object was not easily manoeuvrable and being fully aware of the existence of prevailing winds, it is difficult to invoke force majeure.

In response to a nation's sovereignty violation, the country had a right to proportionately respond to a threat above its territory and to its national security, and "international custom and the principles of law do not exclude the use of force" to address such threats.

To prevent further such incidents, particularly ones that may contribute to the heightened tensions between China and the US, ICAO should adopt a resolution clarifying the rights and obligations of states in relation to the use of civilian balloons and/or airships. From the perspective of aviation and international law, it is hoped that all parties concerned behave in accordance with international law and avoid actions and reactions that intensify the situation.

The Writer is an Ex-Squadron Leader, Bangladesh Air Force, and now an Advocate, Supreme Court of Bangladesh.