Lurking about the presidency in the guise of secretary of state, America's chief diplomat has embarked upon a mistake that someday may rival Dean Acheson's exclusion of Korea from the Pacific defense area, or April Glaspie's muddled words to Saddam Hussein (which suggested the U.S. would countenance Iraq's 1990 invasion of Kuwait). At a regional meeting in Hanoi in late July, Hillary Clinton unveiled an initiative, the subtext of which is an attempt to forge a defensive alliance along the maritime perimeter, with nations such as Vietnam and the Philippines. Like her predecessor Acheson, Mrs. Clinton seems averse or blind to military analysis. Her inevitably stillborn South China Sea initiative is showy diplomacy that may lead either to a military clash with China or, more likely, a ratification of China's aims as the United States lets its implied guarantees die on the vine.

China's assertions in regard to the potentially oil rich and strategically important South China Sea are patently absurd. Based upon the questionable ownership of uninhabited rocks and shoals, some of which do not rise above water and others roughly the size of a Volkswagen, it claims an area almost as large as the Caribbean Basin and as far as 1,800 miles from its nearest shoreline.

By linking America's national interests to those of the coastal states thus insulted, the secretary's recent comments are commendable but insufficiently backed. China above all is sensitive to "paper-tigerism" and ready to challenge it, especially in regard to its essential interests and where the balance of applicable power is swinging in its favor. A naval battle in which China has the upper hand? Do we not have the most powerful military in the world?

We do, but strategic appraisal must not be one-dimensional. Although decisive to some, that this country spends more on defense than the next X countries combined is irrelevant to things such as the scope of its commitments, personnel costs, the willingness or reticence of allies, purchasing power parity; force structure, asymmetrical advantage and disadvantage, domestic politics, strategical genius, its absence, and many other factors including not least geography.

China fought us to a draw in Korea more than half a century ago. In Vietnam we stayed our hand for fear of drawing it into the battle, when its primitive navy was not even a tenth the size of ours, it had no nuclear weapons that could threaten us, and the Western Pacific was an American lake with a necklace of massive military installations now largely abandoned and an alliance structure we are at present trying to rebuild with words. Whittled down by successive administrations, the big stick now turns on the Obama lathe, pressed against the Gates knife. If present trends merely continue, in five or ten years, when the U.S. will have to decide whether to challenge China's claims or acquiesce, the correlation of forces will have shifted much more to China's advantage.

Though China hungers for the oil in these waters that break upon its coast and have long been the subject of its perfervid declarations and domestic propaganda as a kind of oceanic Tibet, it is hardly likely that an American president in his right mind would go to the brink in the South China Sea. In 1996, during the Taiwan crisis, Secretary Clinton's husband cautiously or timidly kept naval forces east of Taiwan, as at China's behest President Obama keeps a carrier strike group out of the Yellow Sea despite the North Korean provocation there.

China's chief advantage in the South China Sea is the force-multiplying proximity of its land-based air and naval power. Without fully developed expeditionary rights and facilities in surrounding states to negate this (i.e. the '60s redux), the U.S. would be forced to strike airfields and ports in China itself, meaning full-scale war. The alternative would be a war of attrition against swarms of China's planes and submarines.

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With no naval or air forces to speak of, and in fear of china's reprisals, our putative allies in the area might be reluctant to host land-based American aircraft. We would have to risk dispatching the bulk of our shrinking carrier fleet for the sake of deploying at most 350 fighter and attack planes against four or five times that number (by then including China's 5th-generation equivalent to our F-22 that we just nipped in the bud) rising from the giant, unsinkable aircraft carrier that is China itself. American hunter-killer submarines can decimate the Chinese submarine fleet, but by no means necessarily before—in conjunction with air and surface assaults, and terminally guided ballistic missiles—it can sink American carriers.

Distances and resupply are decisive. The Paracel Islands, a likely flash point, are 14,000 miles from Norfolk; 8,000 from San Diego; 6,000 from Pearl Harbor; and 2,300 from Guam; but only 200 from the Chinese base at Yu-Lin. China's Guangzhou military region is rich in dispersed bases that if they are vulnerable to attack are no more so than our far fewer and more remote bases in the Western Pacific. China's nuclear deterrent even now makes nuclear weapons immaterial to such a confrontation, no matter which way it goes. And if the United States, suffering losses such as have not been sustained since World War II, was seeming to prevail, the mystery—for some—of China's indulgence toward super-reckless North Korea would come clear as the North invaded the South and, with little in reserve, we turned impotently to the fate of millions of Koreans and the 27,000 American soldiers in their midst.

As Chinese naval, air, and nuclear power rapidly grows and ours diminishes, the Secretary Clinton's recent opening is an invitation to China to gamble on odds moving in its favor. Although the U.S. military has come up with an elegant, workable strategy to counter this dynamic, it cannot succeed if the shrinkage of aircraft and ship numbers, ballistic missile defenses, foreign basing, flying hours, anti-submarine warfare, and the defense industrial sector continues apace.

We must not retreat from making demands that are reasonable and just, but we must be properly equipped to see them through. Diplomats should know this, and that in the relations between rival states little is more dangerous than hollow talk, except perhaps hollow talk that in the uninformed imagination of the speaker appears rock solid.

An earlier version of this essay appeared in the Wall Street Journal.