During my trip to China in late April, numerous officials and scholars there exuded confidence that relations between the mainland and Taiwan are about to improve dramatically. There is some justification for their optimism. The March election of Kuomintang Party (KMT) nominee Ma Ying-jeou as Taiwan’s president promises to greatly reduce tensions in the Taiwan Strait — at least for the next few years. Mr. Ma’s landslide victory was a rebuke of the pro-independence Democratic Progressive Party, which had held the presidency the past eight years and seized every opportunity to push a separatist Taiwanese identity no matter how much that strategy provoked Beijing.

Whether the promise of reduced tensions is fulfilled, however, depends almost as much on the policies that Washington adopts as it does on measures that Taipei and Beijing adopt. The United States must be careful not to do anything that might, however inadvertently, torpedo the cautious rapprochement between Taiwan and the mainland. The rule for U.S. policy should be the equivalent of the Hippocratic Oath in medicine: First, do no harm.

The greatest danger is that the Bush administration in its final months in office might push forward with a proposal for a new arms-sale package to Taiwan. The Taiwanese military is especially interested in purchasing advanced versions of the F‑16 fighter. In addition to the obvious economic incentives to approve such a sale, America’s presidential election cycle might create a potent incentive. A sale of additional F‑16s could be crucial in deciding the electoral votes in the battleground state of Missouri. It was precisely such a consideration that influenced the decision to approve an F‑16 sale on the eve of the 1992 presidential election.

Beijing could make it far more difficult for Washington to push for a new arms sale if Chinese officials move rapidly to offer conciliatory gestures to the new Taiwanese government. Most important, China should announce an immediate freeze on the deployment of missiles across the strait from Taiwan. The PRC has at least 1,000 missiles already in place, and some estimates put the figure at nearly 1,400. Yet it continues to deploy missiles at the rate of several dozen per month.

For years, Washington has cited the missile buildup as the principal reason for offering sophisticated weaponry to Taiwan. American officials argue that the United States is merely leveling the playing field to discourage Beijing from considering the use of force against the island. By announcing a freeze, along with a statement that China is prepared to reduce the number of missiles to pre-2000 levels if Mr. Ma’s new government takes meaningful steps to improve cross-strait relations, Beijing could largely neutralize that rationale.

Unfortunately, Chinese officials seem disturbingly complacent, apparently believing that they can consider such a measure at their leisure. Before they take any action, they want to be certain that Mr. Ma’s conciliatory rhetoric is matched by deeds. (He took office on May 20.)

That could prove to be a tactical blunder. If the missile deployments continue, they will provide the perfect justification for American hawks who want to back pro-independence forces in Taiwan and adopt a more hard-line policy toward Beijing.

But if Washington responds to the continuing missile buildup by offering a new arms-sale package, it will damage, if not destroy, the hopes for a reduction in cross-strait tensions. Beijing would react very badly to such a sale, and those within the leadership elite who might favor a more accommodating policy regarding Taiwan would be undermined.

I am on record elsewhere as favoring a liberal arms-sale policy toward Taiwan. A robust Taiwanese defense capability reduces the likelihood that the United States will be called upon to use its own forces to defend the island — which is an increasingly risky commitment.

But in foreign policy, as in many human endeavors, timing is everything. This would be the wrong time to thrust the F‑16s or other sophisticated armaments on Taiwan. At the very least, we should wait to see whether the promise of improved relations between Taipei and Beijing comes to pass. Washington should do nothing that might undermine such a development. Indeed, U.S. officials should announce a one-year moratorium on any new arms sales to Taiwan.

At the same time, Beijing needs to seize the initiative and move immediately to reduce the missile threat. If the Chinese continue to pursue a lackadaisical pace, they may find that they have missed an irreplaceable opportunity. The time for action is now.