HONG KONG — Taiwan is an often-overlooked player in the debate over control of the South China Sea, where its emphasis on multilateral negotiations tends to be drowned out by the bold claims of China, which considers Taiwan part of its territory and tries to limit its voice in world affairs.
But after an international tribunal broadly rejected China’s claims to the strategic waterway, Taiwan reminded the world that it, too, had a stake in the sea. It denied the tribunal’s findings soon after they were released, and on Wednesday, it sent a warship to patrol the contested region.
“The mission of this voyage is to display Taiwan people’s resolve in defending the national interest,” Tsai Ing-wen, the president of Taiwan, said in a speech before the departure of the ship, a La Fayette-class frigate. The patrol had already been scheduled, but the ship’s departure was moved up a day after the tribunal’s announcment.
Ms. Tsai said the decision on Tuesday by the tribunal, which was established by the Permanent Court of Arbitration in The Hague, had “gravely harmed” Taiwan’s rights in the South China Sea, which is also claimed in part by Brunei, Indonesia, Malaysia, the Philippines and Vietnam.
On paper, Taiwan and China make the same claims to the South China Sea. The so-called nine-dash line that Beijing uses to claim most of the sea is based on a map issued in the late 1940s by China’s then-Nationalist government, which fled to Taiwan in 1949 after losing a civil war to Mao Zedong’s Communists. Since then, Beijing and the government in Taiwan — the Republic of China, as it is formally known — have based their claims on the line, which the tribunal concluded had no basis in law.
But in recent years, Taiwan has hedged its support for the line and emphasized that its claims were based on land features in the South China Sea, Lynn Kuok, a nonresident fellow at the Brookings Institution, said in a 2015 paper. Under the United Nations Convention on the Law of the Sea, claims to bodies of water must be based on adjoining land.
“There is a basic principle in the Law of the Sea, that land dominates the sea,” Ma Ying-jeou, the president of Taiwan at the time, said in an interview with The New York Times in 2014. “Thus marine claims begin with land.”
The most severe blow to Taiwan’s claims in the tribunal’s findings, analysts and government officials said, was its declaration that Itu Aba, the largest land feature in the South China Sea, was not an island that could sustain human habitation or economic activity. Taiwan has controlled the 110-acre Itu Aba, also known as Taiping Island, since 1956.
In recent months, Taiwan has actively promoted its presence on Itu Aba, inviting journalists and scholars on inspection trips. Mr. Ma visited shortly before he left office in May. The tribunal’s declaration that it is a rock, not an island, means that Taiwan is entitled to a territorial sea extending for 12 nautical miles around Itu Aba, but not a 200-nautical-mile exclusive economic zone.
China, which considers Taiwan to be part of its territory with which it must eventually be united, has largely backed Taiwan’s activities in support of its South China Sea claims. Beijing views Taiwan’s position in the South China Sea as bolstering its own argument that there is one China, to which both the mainland and Taiwan belong.
“Chinese people across the strait are duty-bound and obliged to jointly preserve the ancestral land of the Chinese nation,” Lu Kang, a spokesman for the Chinese Ministry of Foreign Affairs, said on Tuesday in a statement responding to Taiwan’s criticism of the arbitration. And Chinese news outlets broadcast Ms. Tsai’s announcement of the navy patrol — an unusual amount of attention for a leader who has faced criticism from Chinese officials and state news media in recent weeks over her unwillingness to express support for the idea of “one China.”
The Ministry of Foreign Affairs of Taiwan also objected to not having been formally invited to present its views to the tribunal and complained that the decision referred to it as the “Taiwan Authority of China.”
“This inappropriate designation is demeaning to the status of the R.O.C. as a sovereign state,” the ministry said, using an abbreviation for the Republic of China.
That aspect of Taiwan’s criticism was not widely acknowledged by China, however. Beijing tries to minimize Taiwan’s international recognition or participation in bodies that would elevate the island’s status and sovereignty. Such pressure has increased under the leadership of Ms. Tsai, who is far more wary of cross-strait relations than Mr. Ma was.