Captain (Retired) Andy (Shichen) Tian, a senior research fellow, is founder and president of the Global Governance Institution and director of the Center for International Law of Military Operations in Beijing. He is also a China Forum expert.

Long before former US House Speaker Nancy Pelosi’s high -profile visit to Taipei, the Taiwan issue had been deeply, frequently and regularly debated by US government ofcials, and those in Congress, academia and think tanks.
The latest discussion focuses on a possible visit by newly elected House Speaker Kevin McCarthy and the Defence Department’s military preparations for the consequences of such a visit.
But as they hotly debate the scenario of an inevitable US military intervention in a hypothetical cross -strait conflict, they forget to discuss – or they turn a blind eye to – the legality and justness of such an intervention. Yet they never seem to forget to criticise the Chinese government for its policy of peaceful reunification with Taiwan while not abandoning the right to use force.
What legal basis does the US have, under international law, to intervene militarily in a cross -strait conflict? To answer this, we need to examine the reasons for a possible US military intervention.
Think-tank dialogues with US experts show that about 80 per cent cite the protection of democracy. Another 15 per cent cite the Taiwan Relations Act, although this is merely US domestic law. The other 5 per cent frankly admit that, strategically, geopolitically and militarily, the US cannot aford to “lose” Taiwan.
The critical issue is whether these three categories of argument hold water under international law. This, as reflected in the UN Charter and international custom, only prescribes two scenarios for the legitimate use of force: UN Security Council authorisation or the right of self-defence. None of the three US categories of defence falls into either of the two scenarios for the legitimate right to use force.
Neither protecting democracy nor implementing domestic law is a lawful exception to the general prohibition against the use of force. If they were, any country could freely use force by claiming to be protecting democracy or through the enactment of a domestic law.
And while no one seems to question the legality of the use of force by the US in case of a cross -strait conflict, a motherland using force to take back its rebellious province is termed an act of “aggression” or an “invasion” .
Although those two words are diferent in English, there is only one word for both in Chinese – qin lue (侵略). This is a weighty word and people tend to equate it with aggression.
But the term “aggression” has a specific meaning in international law, whether in the resolution on the “definition of aggression” adopted by the UN General Assembly on December 14, 1974, or in the amendment to the Rome Statute on the crime of aggression, adopted by the member states of the International Criminal Court on June 12, 2010. The determination of an act of aggression and the crime of aggression are strictly regulated.
In accordance with the provisions of the two international instruments, an “act of aggression” refers to the use of force by a state against the sovereignty, territorial integrity or political independence of another state or in any other manner contrary to the UN charter.
The one-China principle has been confirmed by the international community through UN resolutions. It has also been recognised at the bilateral level by most sovereign nations, including the United States, which conducts diplomatic relations with China. Since Taiwan is not a country but part of China, even if the Chinese central government uses force to restore its sovereignty over Taiwan, it would be a sovereign act rather than one between states.
How could that be taken as an act of aggression under international law? If the US were to intervene militarily, it would be the aggressor, violating the UN’s fundamental principle against the use of force in international relations, in invading Chinese territory.
Acquiescence to a potentially unlawful US military intervention while questioning the Chinese government’s legitimate right to use force only adds to the already rampant tolerance of American exceptionalism in international law.
This has not only led to questions about the moral integrity of the West, but has also destroyed the collective security system enshrined in the UN Charter, which was designed “to save succeeding generations from the scourge of war, which twice in our lifetime has brought untold sorrow to mankind” .
The US military, in shooting down an unmanned Chinese civilian airship that had ended up in US airspace due to force majeure, has set another dangerous precedent, violating the fundamental principle of the UN Charter on the prohibition of the use of force.
Even if the US wanted to neutralise the balloon, it should have employed its law enforcement agencies to carry out the mission. Given that China’s foreign ministry has repeatedly said the balloon was an unmanned Chinese civilian airship, America’s use of its military instead of civilian law enforcement agencies is a clear violation of Article 2 (4) of the UN Charter.
This states, and we hope the US takes note, that: “All members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state. ”

(This article was first published on South China Morning Post on Feb.18, 2023.)