“War is God’s way of teaching Americans geography,” wrote Ambrose Bierce, whose bitter insights were shaped in large part by a terrible war. As a soldier in the Civil War, Bierce witnessed scenes of slaughter over what was, in important ways, a fight to control rivers. You can read it in the names of the great Union armies: the Army of the Ohio, the Army of the Potomac, the Army of the Tennessee, the Army of the Cumberland. The battle over slavery was also a battle to preserve free commerce from east to west on the Ohio River and from north to south on the Mississippi.

A defining question for the coming century is whether Americans can learn geography before it drags us into another potentially disastrous conflict. The place to start is the South China Sea.

Through this crowded waterway passes roughly one-third of the world’s international shipping, worth some $5 trillion per year. From its fertile fisheries comes approximately 10 percent of the world’s annual catch. The sea is the lifeline of multiple major economies, including China, Japan, South Korea, Taiwan, Indonesia, Malaysia, the Philippines and Singapore. At its eastern edge, the sea connects Asia to the Americas; at its western edge, to the Middle East and Europe. Daniel Yergin, author of “The New Map” — a tour de force of geopolitical understanding — sums it up thus: “Those waters are … fraught with risk.”

Bai Meichu, an early-20th-century Chinese geographer, appreciated the power of his field. “Building the nation is what learning geography is for,” he explained. In 1936, he demonstrated that maxim by publishing a map of the South China Sea. Along with observable coastlines to mark national boundaries, Bai’s map included a theoretical line of nine dashes reaching out from China to scoop up the atolls, reefs and shoals speckling the sea — and, by claiming these, to assert control of surrounding waters.

Known as the “nine-dash line,” this cartographic blitzkrieg has come to be official Chinese policy. Beijing claims sovereignty over nearly the entire sea. And thanks to the nation’s extraordinary economic growth, China is gaining the naval and political power necessary to back up its claims.

Over the past dozen years, the South China Sea has been the scene of nearly constant friction. In December, while Americans were distracted by holidays and an attempt to hijack the election, China angrily charged the United States with violating its territory and boasted that its navy had chased a U.S. destroyer, the USS John S. McCain, out of disputed waters claimed by China east of Vietnam.

By all indications, the incident was entirely ginned up. The McCain was engaged in a “freedom of navigation operation,” in which the United States asserts rights of passage — not only for U.S. ships but for vessels from all nations — by sailing through international waters. Navy brass insisted the ship wasn’t expelled. It was just passing through.

But the timing and belligerence of China’s claim is an omen of danger ahead. While Americans were involved in a domestic political crisis that would produce, days later, a mob assault on the Capitol, China bragged to the world that it had sent the U.S. Navy packing. Beijing sees opportunity in our overheated politics.

China’s many neighbors — and the world — are relying on the United States to maintain free passage through this critical waterway. China wants the United States out because that would tip the balance of regional power to China and put Beijing in a position to assert authority over Taiwan.

God forbid we need a war to learn the importance of this hot spot. Our best hope to prevent that dreaded outcome is to build a solid international front in favor of an open sea. The mechanism exists: After decades of argument, the Senate must wake up to a changed reality and ratify U.S. membership in the U.N. Convention on the Law of the Sea. Two generations of conservative senators have objected to the treaty as an over-broad limitation on U.S. sovereignty. That was fine when the U.S. Navy was the only game in town. But China is building a navy capable of competing with the U.S. fleet, which means the time has come for the United States to secure as many partners and allies as possible.

U.S. ratification of the Law of the Sea treaty — formally aligning U.S. policy with the rest of the international community — would make clear that the rising conflict in the South China Sea is not simply a power play between China and the United States. It’s a confrontation between China and the world — a world that depends on a free and open South China Sea.

Geography teaches that. Nine dashes sketched on an old map cannot undo the lesson.

Read more: