The question China watchers asked immediately after President Carter's Dec. 15 announcement of Sino-American normalization of relations was: Why now? The most plausible answer, especially in light of recent developments, perhaps has more to do with the rainly season in Cambodia than a potential U.S.-China trade bonanza.

Throughout October and November, intelligence reports from the Vietnam-Cambodia border area indicated a massive buildup of arms, troops and ammunition in the Parrot's Beak region. Although the heavy rainfall produced a lull in the border skirmishes, the startling implications of these reports were well understood in Peking: Vietnam was preparing for a large-scale conventional assault into Cambodia when the rainy season ended. The debate among the Chinese leadership was not whether Hanoi would move after the rain, but rather what the outcome of the attack would be.

Leaders in Peking viewed the Vietnamese buildup as a direct threat to their security and foreign policy in Southeast Asia. China has considered Cambodia an important ally, despite embarrassment over the harshness of the Pol Pot government. In late autumn, Peking unleashed a vituperative propaganda campaign in its press directed against Hanoi (and Moscow); a crescendo was reached on two different occasions. First, the Moscow-Hanoi "friendship and cooperation" treaty signed Nov. 3 was denounced as a military alliance directed against China and not as an economic development agreement, as stated by the Soviet Union. The treaty includes a provision calling for cooperation in case of threat "for the purpose of eliminating the threat and taking appropriate and effective measures to safeguard peace and security." The treaty earned Vietnam the title "Cuba of the East" in the Chinese press and firmly implanted in Peking the belief that Vietnam is but a client state for Soviet imperialism in southern Asia.

Second, the announcement in a Radio Hanoi broadcast on Dec. 3 of a "Cambodian National United Front" (complete with radio station and news agency) evoked vitriolic invectives against Moscow and Hanoi from the Chinese press. To Peking, the "Front" is a flimsy pretext for Vietnamese aggression. The Chinese were most alarmed, however, by the fact that the rain was due to end just three weeks after this announcement.

The message in Peking was clear: Hanoi would move around Christmas. The aim of a hastily convened politburo meeting in Peking in early December was to propose action sufficient to deter the fall of the Pol Pot government. The Sino-American normalization option provided Peking with what it hoped would be a strategic countermove.

Peking's analysis of the deteriorating situation on its southern border must have been so compelling to President Carter and his advisers that they signed an agreement after a 100-hour diplomatic flurry. It is perhaps testimony to the "geopolitical" age in which we now live that Washington harkened to Peking's alarms. The administration was even willing to antagonize conservative Senate votes needed this winter for a SALT agreement.

While American leaders have emphasized the commercial benefits of normalization, the Chinese have spoken of the agreement almost exclusively in strategic terms. Vice Premier Teng Hsiao-ping even used the Chinese word for "alliance" recently when referring to the pact -- a term that surprised China specialists.

Yet the Sino-American link clearly failed to deter the fall of Phnom Penh, as the Chinese hoped it would. The events of the last few days represent a serious setback to the post-Mao leadership. To conclude the normalization agreement with the United States, Peking hastily agreed to permit American arms sales to Taiwan, to halt the shelling of the off-shore islands and to issue unprecedented statements of peaceful intentions concerning reunification with the mainland. Recent Chinese troop redeployments to the Sino-Vietnamese border, far from inhibiting Hanoi's thrust into Cambodia, have been met with Soviet troop movements along the Sino-Soviet border in the northwest.

Clearly, Peking misjudged Moscow's "control" over Hanoi and, thus, the deterrence value of a Sino-American normalization agreement to Vietnam's ambitions in Cambodia. Peking conveniently dismisses the more than 400-year history of animosity between Vietnam and Cambodia. The new leadership in Peking still clings to the Maoist theory that the world is dominated by two competing imperialist superpowers (with the Soviet Union the more feared), each with client states acting explicitly on its behalf.

While a Sino-American normalization agreement might have deterred the Soviet Union from encouraging a "Cuban-type" foray into Cambodia, the new pact has evidently done little to deter the strong-minded, independent leadership in Hanoi from its own form of imperialism. Americans learned about Hanoi's truculence the hard way. Peking may be in for a similar education.