Last Monday, an American littoral combat ship, the Fort Worth (pictured below), sailed just over 12 nautical miles from some of the artificial islands that China has built from reclaimed land in the South China Sea.  While the U.S. Navy has conducted such transits before to ensure freedom of navigation through the region, it was the first time that a U.S. warship came so close to Chinese-held islands.  The transit was part of a stepped-up American effort to deter China from asserting its claims in the Spratly Islands too aggressively.  That effort has included openly questioning China’s maritime claims in December 2014 and encouraging Japan to take a bigger security role in the region earlier this year.

Last week, the United States also revealed that it is considering sending its ships and surveillance aircraft within 12 nautical miles—the internationally-recognized territorial zone around natural islands—of China’s newly-built islands, which the U.S. does not regard as natural.  If that happens, an incident between U.S. and Chinese forces may well take place.

Currently the Fort Worth, based in Singapore, is the only U.S. warship whose homeport is anywhere near the Spratly Islands.  While the U.S. Navy plans to eventually base four littoral combat ships in Singapore, when that might happen is not yet known.  In the meantime, America’s thin grey-hulled line in the South China Sea will be very thin indeed.

Thus, the Pentagon must consider how it would respond to a crisis there if one should occur.  Broadly speaking, the U.S. Navy expects to flow forces into the region from other areas around the world.  But to reach the South China Sea, those forces would have to pass through or near a number of choke points.  Those choke points would be natural places where China could intercept U.S. forces.

The U.S. 7th Fleet, based in Japan, would be the closest reinforcements that the United States could dispatch.  It would also be the most susceptible to Chinese interception.  To reach the South China Sea it would likely sail down the eastern flank of the Ryukyu Islands and through the Luzon Strait.   On the way it would pass the Miyako Strait, through which submarines and warships of China’s East Sea Fleet are known to sortie into the Pacific Ocean.  Then, as the U.S. fleet passes through the Luzon Strait, it would face the full brunt of Chinese naval and air forces stationed along China’s southern coast, including the main bases of China’s South Sea Fleet at Zhanjiang and Yalong Bay.  While the Los Angeles-class nuclear attack submarines sailing underwater from Guam may avoid Chinese air power, both U.S. surface and subsurface forces would likely encounter Chinese submarines in the confined spaces of the Luzon Strait and in the waters near the Paracel Islands.

The U.S. 5th Fleet, normally operating near the Persian Gulf, would be the next closest source of reinforcements.  Its principal challenge to reach the South China Sea would be to sail unfettered through the long and narrow Malacca Strait.  There, Singapore’s capable navy and air force could play a vital role in keeping watch for Chinese aircraft and submarines, even if it did not want to directly involve itself in the dispute.

The last forces to arrive might deploy from as far as Hawaii or the U.S. West Coast.  They would be largely drawn from the U.S. 3rd Fleet.  Those forces may choose to eschew the Luzon Strait altogether and support operations in the South China Sea from the Sulu or Celebes Seas.  There, they could operate in relative safety, though still within range of Chinese anti-ship ballistic missiles.  At least the mountains of Palawan Island would degrade the ability of Chinese land-based high-frequency direction-finding equipment and over-the-horizon radar to accurately target U.S. forces.  Resupply, particularly of ordinance, could be routed by air through Zamboanga (where U.S. Special Forces have operated for about a decade) or by ship through Davao or Koror.

All of which is to say that the success (or failure) of an American response to a crisis in the South China Sea depends, in no small part, to what happens in and near those straits, and that those straits should be very much present in the minds of U.S. naval commanders.  They also remind us of the abiding importance of geography, even in naval warfare.