During Gorbachev's 1988 speech in the UN, why did he say that "force no longer can . . . be an instrument of foreign policy"? What implications did this have for the Soviet bloc?

By the time he gave his speech to the UN, Gorbachev had been carrying out his flagship policy of glasnost, or openness, for three years, and his speech reflected this theme. Gorbachev's new policy had inadvertently encouraged the growth of nationalist sentiments among the non-Russian peoples of the Soviet Union, such as Estonians and Lithuanians, as well as in Eastern Bloc countries such as Poland and Romania. Although this was an unintended consequence of glasnost, Gorbachev knew that, once the nationalist genie was out of the bottle, it would be hard if not impossible to put it back again. So he tried to accommodate nationalism within the existing Communist system.
In retrospect, Gorbachev's approach can be seen as doomed for failure; but there's no doubting his genuine belief that the Soviet Union could be kept together while he continued to pursue the policy of glasnost. In former days, Soviet leaders had readily resorted to force to maintain control, as seen by the invasions of Hungary in 1956 and Czechoslovakia in 1968. But now things were different. Gorbachev knew that, with nationalist sentiment on the rise throughout the Soviet Union and the entire Eastern Bloc, armed force would no longer be able to impose the Kremlin's will. Gorbachev's very public renunciation of force, combined with the ongoing policy of glasnost, emboldened nationalists in the Eastern Bloc, encouraging them to believe that the Soviet Union, though still a repressive, one-party state, would not use extreme measures in stifling the growing demand for national independence.