Chairman Hua Kuo-feng's remarkable trip to Romania and Yugoslavia prompted a revealing tidbit of news the other day.According to a dispatch in this newspaper, Romania leaders - long recognized as a crafty lot - decided this was a good time to explore anew the limits of their strange alliance with Moscow by inviting Hua to make anti-Soviet noises from the Soviet hinterland in Romania. It was a good time, the Romanians reckoned, because the Soviet Union is weak at the moment and preoccupied with difficulties it is having in Washington and Peking.

A faltering Soviet Union? The point could be overstated, but there is a kernal of Balkan wisdom in that Romanian observation, one worth noting on the eve of the new diplomatic semester that will open officially on Sept. 5 at Camp David.

As summer ends, the Kremlin's leaders (most of whom have spent recent weeks on the Black Sea coast, not in the Kremlin) cannot be encouraged by the international situation. Nothing has gone particularly well for them lately, except perhaps the decline of the dollar - a fortuitous windfall - and a lot has gone badly.

The worst news of the summer by far was Japan's decision to ignore elaborate threats from the Soviet Union and proceed to sign a treaty of friendship and cooperations with China. This was a cardinal defeat for the Soviets that symbolized the failure of a diplomatic strategy that Moscow has pursued for many years.

At the beginning of this decade it appeared likely that Soviet-Japenese relations would develop into a vigorous economic partnership. Japan then seemed willing to invest billions in the development of Siberia's vast energy resources, and the Soviets seemed eager to share energy with Japan in return for financial and technical help. Political relations were less promising but perfectly adequate, and the Russians fairly assumed that Japan's economic interests would prevent any Sino-Japenese rapprochement at Moscow's expense.

In the course of the 1970s all that has changed Japan obviously sees much brighter prospects in China now than in the Soviet Union. Most of the ambitious plans for Japenese investment in Siberia have fizzled. Japan agreed to sign the new treaty with China, knowing that the Chinese and the Soviets would both regard that acts as an anti-Soviet gesture.

So the Soviets have probably lost a potentually useful relationship with a rich and powerful neighbor - and lost it to their most hated enemy, which of course also happens to be a neighbor.

The news from Africa, one of last semester's most active fronts, must also look grim in Moscow. The Organization of African Unity came uncomfortably close to condemning Soviet and Cuban involvement in Africa at its summer summit meeting. The Namibia situation, once a promising source of tension and turmoil in Southern Africa, appears now to be succumbing to a sensible process of negotiations. The Angolan government, once seen as a staunch ally of the socialist camp, has been playing up to the Americans and even dealing civilly with the Mobutu regime in Zaire in an effort to stabilize their common border.

Africa remains rich in potential for the machinations of Soviet diplomacy, but any end-of-summer assessment in Moscow would have to conclude that the potential remains unfulfilled, and that new dangers of failure have materialized.

The summer has also brought an unexpected turn of events in the Middle East that cannot sit well in Moscow. When the vacation season began Egyptian President Anwar Sadat's peace initiative appeared to be expiring, raising a range of new possibilities for Soviet diplomacy. Now, suddenly, the initiative has been revived dramatically by the Camp David summit. Of course the men in Moscow can hope for total failure, but they also must recognize the possibility of significant success at Camp David.

Relations with Washington have continued to be bad, with new complications arising during the course of the summer. The Soviets have now provoked a tougher American attitude toward the sale of high-technology goods to the U.S.S.R. and also set off a furor in the American intellectual community that has already damaged Soviet-American exchange programs and threatens to terminate some of them.Both are the result of the unseemly crackdown on dissidents last spring and early in the summer.

Outsiders may doubt that those reactions worry the Soviets, but recent evidence suggests that they do. Under substantial pressure, the Soviet authorities backed down dramatically in the case of the two American reporters they had accused of "slander" for writing stories on a group of dissidents who had challenged the veracity of Soviet television.

A few weeks ago the Soviets seemed bent on a course of action that would have ended at the least in expulsion of the two reporters, but the Carter administration made clear its intention to retaliate sharply if they were expelled, and the case against the two has now been closed. Craig Whitney of The New York Time and Harold Piper of The Baltimore Sun did pay unprecedented fines in the case, but they did not print retractions ordered by the Soviet court, and the Soviet authorities' obvious desire to intimidate the Western press into less active coverage of dissident news has apparently provoked an opposite reaction.

The emergence of a vigorous new Chinese diplomacy must be the most disquieting development of recent months for the Soviet Union. China has strengthened its relations with the United States and set off a flurry of activity that could end in full diplomatic relations sometime next year (at least, that is the private prediction of senior American officials). The Chinese have sealed the Japanese treaty, and now they have posed a discomfiting challenge to Soviet power in the traditional Soviet sphere of influence by sending Chairman Hua on his trip to Romania and Yugoslavia. At the same time, the Chinese government under Teng Hsiao-ping has set off on a pragmatic new course that Moscow must find exceedingly ominous, because it suggests that China is abandoning ideological purity in a new effort to make itself a modern power.

In sum, the world has given the old men in Moscow a good deal to worry about, and at an inopportune time when they already had worries enough of their own. This is a succession season in the Kremlin, when the leaders must be preoccupied above all with the health of President Leonid Brezhnev and the designation in the relatively near future of a new leader.

So the Romanians who decided this might be a good moment to tweak the ear of the Russian Bear had a point that deserves some attention in other world capitals, beginning with this one.