THE UNITED NATIONS has just concluded a decade-long effort to condemn terrorism.

Why was it so hard to speak out clearly against hijacking airliners, bombing buses and murdering civilians? You already know: a leading perpetrator of these crimes, the Palestine Liberation Organization, is a potent force in the General Assembly, and it wrapped them in the cloak of a struggle for self-determination against Israel and, by extension, the United States.

What broke the PLO's intimidation of its customary protectors, the communist-Third World majority? The PLO is widely recognized now as a principal actor in international terrorism. Its role in hijacking the Achille Lauro was merely its most conspicuous recent embarrassment, so much of one, in fact, that Yasser Arafat was sbsequently compelled to condemn acts against innocent and defenseless people, unless -- his cynical exception -- they are Israelis.

As terrorism has increased, moreover, so have the nations victimized by it. So long as Israelis were the main target, many other nations bought the PLO's argument that the "underlying cause" of terrorism is Israeli oppression. But how, then, to explain such deeds as the kidnapping of four Soviets, and the murder of one, by crazies in Beirut? It turned out the PLO was popularizing not so much the Palestinian cause as the terrorist example, teaching its methods to those with other purposes, extending its reach and hurt, in an awful sense secularizing and democratizing it.

Thus was created a constituency ready to say, with no ifs, ands or buts, that terrorism is a bad thing. Cuba tried, of course, to muddy the issue by working in a condemnation of "state terrorism," General Assembly-ese for Israel and the United States. The British and the Americans, furthering a campaign t get other nations to deepen their commitment against terrorism, replied that there was no shortage of legal instruments condemning questionable acts committed by states, but that what was needed here was an instrument to counter terrorism committed by groups and individuals -- a phenomenon otherwise hard to pin down by law. In committee, Cuba's proposition got one vote: Cuba's.

The whole Assembly then condemned "as criminal all acts, methods and practices of terrorism wherever and by whomever committed . . ." "The adoption," said a U. N. document, "was followed by applause from the representatives." It was earned. Secretary General Javier Perez de Cuellar said this was how the U.N. system was supposed to work: nations uniting in solidarity on universal problems. It is a long way from word to deed. But saying the right word is important and finally, on terrorism, the U.N. has said it.