The near-simultaneous publication of US and Japanese reports analyzing Chinese military power has riveted Taiwan's attention on strategic conditions in the Strait. Both reports documented China's swift military buildup. While the Pentagon hedged about Chinese intentions, however, the Japan Defense Agency strongly implied that Beijing was shifting to an offensive stance.

The common wisdom among Taiwanese strategists holds that the military balance in the Strait will tip in favor of China as early as next year. Improvements to Chinese air, naval and missile forces will ultimately negate the advantage in quality the Taiwanese forces have long relied on to deter Chinese military action.

Burgeoning military power may tempt Beijing to settle the cross-strait impasse by force. But "island warfare" endangers not only islanders but land powers that venture seaward.

Taiwan must preserve the military balance if it wants to choose its own destiny. For its part, China should not assume that military superiority guarantees it victory in a trial of arms.

Wars of antiquity bear out the perils of island warfare. Thucydides, the historian of the Peloponnesian War, recounts an event relevant to Taiwan. During its decades-long struggle with rival Sparta, Athens dispatched an embassy to Melos, an island city-state strategically located off the Greek coast, with an ultimatum: submit to the Athenian empire or be destroyed.

Melian leaders balked, but their city lacked military means adequate to fend off an Athenian assault. The Athenian ambassadors sneered at the Melians' appeals to justice, proclaiming that the strong do as they will in world affairs, while "the weak suffer what they must" when their interests clash with those of the strong.

True to their word, the Athenians crushed the island's feeble resistance, put its male population to death, and enslaved the women and children.

For Thucydides, the butchery illustrated what too often happens when one power defies another without the armed strength to protect itself.

"Questions of justice," he warned, "arise only between equals."

Now as then, effective diplomacy rests on a rough parity of hard power as much as on law or abstract ideals. In other words, Taiwan must arm itself if it expects equitable treatment from China.

Still, Thucydides offers China a cautionary tale of its own. A few years before the bloodletting at Melos, an Athenian expeditionary force landed at Pylos, some fifty miles from Sparta, and erected a fort to harass the Spartans in their own backyard. Grasping the danger, Spartan leaders dispatched forces by land and sea to wrest the Athenian outpost from its defenders.

After an initial skirmish, Spartan hoplite warriors invested Pylos by land. Another force landed on Sphacteria, a long, narrow island athwart the harbor mouth, to cut the fort off by sea.

Spartan fortunes soured when fifty Athenian warships appeared unexpectedly, putting the Spartan flotilla to flight "at once," disabling or capturing "a good many vessels," ramming others, and towing away beached vessels abandoned by their crews. Proud Spartan infantrymen were reduced to wading into the surf in a futile effort to recover their vessels.

"The stunning effect and importance" of the Athenian naval attack, notes Yale University professor Donald Kagan, "cannot be exaggerated."

Their expeditionary force blockaded, Spartan leaders sent an embassy to Athens to sue for peace, only to have their overtures rebuffed by an Athenian assembly that was in no mood for compromise. Reinforcements sailed for the island.

Athenian troops overwhelmed and captured the Spartan defenders, who were brought to Athens as hostages.

Pylos humbled Sparta's vaunted land power, underlining the dangers of island campaigning for a land power facing a dominant sea power in its element.

Judging from the Spartan example, time may not be on China's side in a cross-strait war, as the Pentagon report suggests.

Should the US Navy force the Strait after a Chinese amphibious landing, Chinese forces could find themselves isolated and under siege, much like the Spartan hoplites.

The repercussions of defeat could be as frightful for China's international standing -- even its domestic stability -- as they were for Sparta's.

Both Taipei and Beijing, then, should heed Thucydides' wisdom. To discourage Chinese adventurism, Taiwanese lawmakers should set aside the prolonged bickering that has stalled a proposed special arms package in legislature.

They should either approve this package or negotiate another one that better meets Taiwan's military needs.

And China should ponder the lessons of Pylos before resorting to arms.