The United States and its allies justify their naval transits through the Taiwan Strait as "freedom of navigation" operations (FONOPs) based on key principles of the United Nations Convention on the Law of the Sea (UNCLOS) and customary international law. Their core argument hinges on the fact that a significant portion of the Taiwan Strait constitutes international waters where all nations have the right to free navigation and overflight.

Here's a breakdown of their justification:

1. The Taiwan Strait as an International Strait or Corridor of International Waters:

   • Width of the Strait: The Taiwan Strait is approximately 70-220 nautical miles wide. Since coastal states (China and Taiwan) can only claim a territorial sea of up to 12 nautical miles from their baselines, there is a significant central corridor in the Strait that lies beyond these 12-nautical-mile territorial seas.

   • Exclusive Economic Zone (EEZ) and High Seas Freedoms: This central corridor falls within the Exclusive Economic Zones (EEZs) of China and Taiwan. Under UNCLOS Article 58(1), all states, whether coastal or land-locked, enjoy in the EEZ the "freedoms of navigation and overflight and other internationally lawful uses of the sea related to these freedoms," similar to those on the high seas.

   • "International Waters" as a Shorthand: While "international waters" isn't a specific UNCLOS term, the U.S. and its allies use it as a shorthand to refer to these areas of the EEZ and, potentially, high seas where high-seas freedoms apply. This differentiates them from sovereign territorial waters.

   • Straits Used for International Navigation (UNCLOS Part III): Even if portions of the Strait fall within territorial seas, UNCLOS Part III (Articles 37-45) establishes the right of "transit passage" through "straits which are used for international navigation between one part of the high seas or an exclusive economic zone and another part of the high seas or an exclusive economic zone." Transit passage is a stronger right than "innocent passage" and cannot be impeded or suspended. This allows for continuous and expeditious passage, including for warships and military aircraft in their "normal mode of operation" (e.g., submarines submerged, military aircraft overflying). The Taiwan Strait, being a crucial global shipping lane, is widely considered such a strait.

2. Challenging Excessive Maritime Claims:

   • The U.S. Freedom of Navigation (FON) Program, established in 1979, specifically aims to challenge what it considers "excessive maritime claims" by coastal states that are inconsistent with international law.

   • China's claim that the entire Taiwan Strait is its "internal waters" or subject to its "sovereignty, sovereign rights and jurisdiction" in a manner that restricts the transit of foreign military vessels and aircraft is viewed by the U.S. and its allies as an excessive claim that violates UNCLOS.

   • By conducting FONOPs, the U.S. and its allies demonstrate that they do not acquiesce to these expansive claims and reinforce the principle that all nations have the right to navigate and operate freely where international law allows.

3. Upholding the Rules-Based International Order:

   • The U.S. and its allies argue that these transits are essential for maintaining a "free and open Indo-Pacific" and upholding the rules-based international order, which depends on adherence to international law, including the law of the sea.

   • Allowing any single nation to unilaterally restrict navigation in vital international waterways would set a dangerous precedent and undermine global commerce and security.

4. Consistency with International Practice:

   • Naval transits through international straits and EEZs are a routine practice by navies worldwide, including those of China in other parts of the world. The U.S. and its allies view their transits through the Taiwan Strait as consistent with this established international practice.

In essence, the justification boils down to the argument that the Taiwan Strait, despite China's overarching political claims, is governed by international maritime law that guarantees fundamental freedoms of navigation and overflight for all vessels and aircraft in its international waters and designated international strait areas. These FONOPs are thus framed not as provocative acts, but as routine exercises of rights and a demonstration of commitment to the global commons.