IT is rare for interests crucial to two nations to be bound up in the resolution of a crime. Yet the assassination of Orlando Letelier and Ronni Moffitt is such a crime. For the United States, the case is a litmus test of its commitment to oppose international terrorism. For Chile, the case has become a benchmark of the nation's capacity to restore the rule of law after one and a half decades of dictatorship. Fifteen years after the crime, its resolution remains the single most important issue in relations between the two nations. Mr. Letelier, a prominent opponent of former Chilean President Augusto Pinochet Ugarte, and Ms. Moffitt, a US citizen who worked with Letelier in Washington, were killed by a bomb as they drove to work on Sept. 21, 1976. According to testimony obtained from other participants convicted in US courts, the authors of the crime were Manuel Contreras Sepulveda, then director of Chile's notorious secret police, DINA, and his deputy, Pablo Espinoza. Their extradition was sought by the US government as long ago as 1978, and US aid to Chile was suspended until significant progress had been achieved in the case. Yet despite its importance, the case was, until recently, headed for oblivion. US extradition efforts had been rebuffed by Chile's Supreme Court. Military courts charged with investigating the crime have failed to indict General Contreras and Colonel Espinoza in the nearly 15 years since the crime occurred. With Chile's statute of limitations for murder due to expire on Sept. 21, their lasting impunity seemed assured. But the long-dormant case came to life in early July, when Chile's Supreme Court rejected a military prosecutor's motion to bring a final close to the case. Two weeks later, the Court, acting on a request of Chilean President Patricio Aylwin Azocar, designated one of its own members, Adolfo Banados, to investigate the case. On Aug. 26, Justice Banados issued an order prohibiting Contreras and Espinoza from leaving Chile. His efforts could finally bring two of America's most wanted fugitives to justice. The designation of Banados also holds forth the best prospect of bringing to justice the officials who were ultimately responsible for the murder and disappearance of thousands of Chilean citizens. Last February, a commission appointed by President Aylwin to investigate political killings and disappearances committed during the 17-year dictatorship of General Pinochet issued a report documenting more than 2,000 cases of disappearance and murder, most carried out by agents of DINA. While a 1970s amnesty l aw bars prosecution of most of these crimes, the Letelier/Moffitt case is explicitly exempted. A conviction in this case would be a significant step toward reclaiming Chile's proud democratic tradition in the aftermath of the nation's prolonged descent into lawlessness. BUT if the case for prosecution is compelling, the obstacles are formidable. Although Banados has a strong reputation for independence, his decisions would be reviewed on appeal by the entire Supreme Court, which is dominated by conservative members who consistently refused to act on human rights cases during the Pinochet era. The court's vote to designate an investigating judge was close - only 9 of 17 members approved the decision. Most Chilean legal experts say Banados must act to name defendants befo re Sept. 21 to avoid a legal deadline that could bar prosecution. Pinochet, who retains considerable power as Army commander-in-chief, might resist any effort to charge Contreras and Espinoza. In the past he has vowed to oppose any efforts to prosecute "his men" for human rights violations. Banados may thus face personal risks and will look to the Aylwin government for the visible and uncompromising support he will need to pursue a vigorous investigation. But the government, too, will face risks, underscored by Pinochet's willingness to call out the troops when his own will is thwarted, as he did last December during a dispute with the civilian government. While serious, these challenges are not insurmountable. During a recent visit to Santiago, we were told by government officials that it is unlikely that an indictment of Contreras would provoke a major military threat. They assured us that they would continue to press for a thorough investigation. The US must do its part to support the Chilean government's efforts by continuing to voice unequivocally its own determination to see that justice is done in the Letelier/Moffitt case and to cooperate fully with any requests from the investigating judge. The US should also be prepared to renew efforts to extradite Contreras and Espinoza for trial here if prosecution in Chile is blocked. Further, the US should immediately suspend any plans for military aid to Chile, authorized last December after a 14-yea r ban, if the military obstructs the judicial process. The murders of Orlando Letelier and Ronni Moffitt are no more terrible than the thousands of other murders by DINA agents. But a successful prosecution of their murderers would be an important step toward establishing accountability for the wholesale violence committed at DINA's direction and reestablishing the rule of law in Chile. It is crucial that the Chilean and US governments seize this window of opportunity to assure that Contreras and Espinoza are, at last, brought to justice.