If the First Island Chain Is Not a Wall, What Does It Do?

Who drew the first island chain, and is it a border?

It is not a border. The line has no legal standing and appears in no treaty; it simply describes a geographic fact: running south from Kyushu through the Ryukyus, Taiwan, and the Batanes into the Philippines, the islands happen to form a continuous chain.

The term is usually traced to early Cold War US strategic planning, where it described the geography for containing the Soviet Union and China. It is still in use seventy years later, not out of inertia but because the geography has not changed.

Treating it as a border leads to two errors: assuming that crossing it amounts to an invasion, or that holding it amounts to a defence. Neither is right: the chain is a set of terrain conditions, not a line.

Which countries make up the first island chain?

At its core are three: Japan, Taiwan and the Philippines. The northern stretch is Japanese, running from Kyushu down the Ryukyus, which include Okinawa; Taiwan sits in the middle; the southern stretch begins at the Batanes and belongs to the Philippines. Of the three, Japan accounts for the largest share.

Broader definitions take in both ends. The First Island Chain Deterrence Act, a bill introduced in the US Senate in March 2026, defines the chain as the archipelagos “extending from the Kuril Islands and Japan, through the Ryukyu Islands, Taiwan, and the Philippines, to Borneo”, the first line of islands off the East Asian mainland. On that reading, the northern end, the Kurils, is under Russian administration, and the southern end, Borneo, is divided between Malaysia, Indonesia and Brunei.

Nor do the maps agree. Andrew Erickson, a professor at the US Naval War College, has pointed out that the Pentagon’s map leaves South Korea out of the first chain while a Chinese navy book puts it in, and that Chinese maps have the chains join up in Japan and run north across the Sea of Okhotsk to the southern tip of Russia’s Kamchatka peninsula.

The differences lie at the edges, and Erickson calls them matters of nuance rather than fundamental disagreements. The stretch from Kyushu through Taiwan to the Philippines is broadly the same across versions, and it holds the chain’s two main passages, the Miyako Strait and the Bashi Channel.

Why would a string of islands carry strategic weight?

Because the waters between islands can be watched and closed, while open ocean cannot.

A fleet moving from the East or South China Sea into the Western Pacific has only a few usable passages: the Miyako Strait, the Bashi Channel, or the gaps further south among the Philippine islands. They range from a few tens of kilometres to about 250 km across, and modern surveillance and anti-ship capability can keep all of them under continuous observation.

For submarines the point is sharper still. Submerged transit needs depth and room, and a strait’s hydrography, depth, and noise environment strongly affect whether a boat can pass undetected.

So the chain’s significance is not that it blocks but that it makes passage predictable. What is predictable can be planned for, and planning is what produces deterrence.

What makes Taiwan’s position on the chain special?

Because Taiwan sits midway along the chain, between its two main passages: the Miyako Strait lies to its north and the Bashi Channel to its south.

That is also why the waters east of Taiwan have drawn attention in recent years. The strait to the west is heavily militarised and closely watched; the east faces the open Pacific and was long treated as a rear area, as well as the corridor allies use to maintain sea lanes and exchange intelligence.

Once the east becomes a zone of routine pressure, Taiwan’s defensive depth shrinks: space it could once fall back on becomes a second front that must also be held.

And the second island chain?

The second chain runs south from Honshu through the Bonin Islands, the Marianas, and Guam towards Palau. It lies further from the mainland, with wider gaps between the islands, so its role is different: it provides rear basing and depth rather than control of passages.

Guam is the critical node, one of the most important US forward bases in the Western Pacific. That is why missiles that can reach Guam directly change the calculus on the first chain: if the rear base is itself within range, the forward posture has to be redesigned.

Put simply, the first chain determines whether movement can be seen; the second, whether the position can be sustained.