CALL TO ARMS

National Security Decision Directives are top-secret orders dealing with sensitive problems that threaten America's safety. But last week Vice President George Bush openly discussed one directive, signed by President Reagan in April, that will allow the U.S. military to play a more active role in the nation's fight against drug trafficking. Bush, who headed the President's National Narcotics Border Interdiction System, said he was publicizing the order in an effort to make ''every American understand the very real link between drugs and terrorism.'' Bush charged that Nicaragua's Sandinista regime was engaged in the drug trade and that the leftist guerrillas who waged a bloody assault on the Colombia Palace of Justice last year destroyed U.S. extradition requests for Colombia's most notorious narcotics traffickers. ''Now we must convey that when you buy drugs,'' said Bush, ''you could also very well be subsidizing terrorist activities overseas.'' The armed forces have been engaged in the drug war since 1981, when Congress revised the 100-year-old posse comitatus act that prohibited the military from involvement in domestic law enforcement. With the reins loosened, the Department of Defense began providing the Drug Enforcement Administration and other organizations with radar surveillance, communications hardware and help in planning raids on traffickers. What, then, makes the new directive significant? ''The greatest impact may be one of emphasis,'' said Bush Aide Kevin Cummings. ''We now have a forthright identification of the problem as a national-security concern.'' That could encourage local military commanders, who have discretion over what personnel can be assigned to any given operation, to be more forthcoming when assistance is requested by DEA or the Coast Guard. In addition, some observers suspected that Bush, a candidate for the Republican presidential nomination in 1988, had more personal motives in publicizing the directive. ''If you said that politics played an important role in this announcement,'' said a military official, ''you wouldn't find a lot of people objecting at the Pentagon.''