What Is the First Island Chain? China, Taiwan and the Pacific Line
What is the first island chain? How Congress’s researchers define the line from Japan through Taiwan to the Philippines, and why it shapes US-China strategy.
The first island chain is a line of islands off China’s east coast that U.S. defense planners and Chinese strategists both treat as a key military boundary in the Western Pacific. It comes up often in news about Taiwan, Japan’s southwestern islands and the South China Sea. This explainer covers what the term means, which places it includes and why it matters.
What is the first island chain?
The Congressional Research Service (CRS), Congress’s research agency, defines the first island chain as “a string of islands, including Japan and the Philippines, that encloses China’s near-seas region.” In its report on U.S.-China competition in the South and East China Seas, updated May 1, 2025, CRS uses “near-seas region” to mean the South China Sea, the East China Sea and the Yellow Sea.
Taiwan sits near the middle of the chain. CRS adds that “the exact position and shape of the lines demarcating the first and second island chains often differ from map to map,” so there is no single official line.
What is the second island chain?
The same CRS report describes the second island chain as a line that “reaches out to Guam” and encloses both China’s near seas and the Philippine Sea between the Philippines and Guam. The chains are a way of describing distance from the Chinese mainland: the first is the inner ring, and the second is a wider ring that takes in major U.S. bases on Guam.
Why the first island chain matters
According to CRS, Chinese bases and forces in the South China Sea would add, in a conflict, to a network of anti-access/area-denial (A2/AD) capabilities “intended to keep U.S. military forces outside the first island chain (and thus away from China’s mainland and Taiwan).” CRS lists other ways that control of the near seas could help Beijing day to day, including announcing an air defense identification zone over the South China Sea or enforcing a maritime exclusion zone around Taiwan.
For Taiwan, the stakes are direct. A CRS In Focus on Taiwan, updated Oct. 20, 2026, quotes Chinese leader Xi Jinping as saying the PRC will “strive for peaceful reunification” but “will never promise to renounce the use of force.” The same report notes that the 1979 Taiwan Relations Act commits the United States to make defense articles available to Taiwan but does not say whether U.S. forces would defend the island.
How Congress is using the term
The phrase now appears in legislation. The First Island Chain Deterrence Act (S.4223), introduced on March 26, 2026, by Sen. Michael Bennet (D-Colo.) with Sen. Pete Ricketts (R-Neb.) as a cosponsor, would require the Government Accountability Office to report on how delays in arms deliveries to Japan, Taiwan and the Philippines affect the Pentagon’s ability “to build and sustain a strong denial defense in the First Island Chain.” The latest action listed on Congress.gov is the bill’s referral to the Senate Foreign Relations Committee.
What it means
Analysis: The concept explains much of the region’s current military activity. U.S. missile deployments to islands such as Yonaguni, Japanese exercises in its southwestern islands and Chinese pressure on Philippine outposts all sit along the chain. When officials talk about “denial defense,” they mean using those islands to make it harder for an opponent’s ships and aircraft to pass through.
Follow the latest developments on the China-Taiwan tensions news hub and the daily PLA activity tracker.
Sources
- Congressional Research Service: U.S.-China Strategic Competition in South and East China Seas (R42784)
- Congressional Research Service: Taiwan: Background and U.S. Relations (IF10275)
- Congress.gov: S.4223, First Island Chain Deterrence Act
