The arrival of U.S. military forces in Bolivia to help with a major assault on cocaine processing facilities this week was carried out under a secret directive President Reagan approved April 8 at the urging of Vice President Bush and following Defense Department objections, officials said yesterday.

The Bolivia action -- involving a U.S. Galaxy C5A cargo plane, six Black Hawk helicopters and a Hercules C130 carrying trucks, jeeps, radio equipment and field gear -- was the first operation conducted under the new directive, officials said.

The directive permits the use of U.S. forces in helping other nations fight narcotics trafficking, and also authorizes an expanded use of American intelligence, including electronic eavesdropping, to combat the drug trade in other nations.

Bush, describing the Reagan directive June 7 in Houston, told reporters that it "significantly improves" the ability of the defense and intelligence agencies "to support our war on drugs."

"For the first time through the directive, the U.S. government specifically states that the international drug trade is a national security concern because of its ability to destabilize democratic allies through the corruption of political and judicial institutions," he said then.

However, officials said that the military had opposed using American forces for combating drugs on grounds that it would take manpower and equipment away from other tasks. Defense Secretary Caspar W. Weinberger reportedly resisted the idea, but went along after Bush made a personal appeal to Reagan, officials said. Reagan, who is considering a new initiative aimed at combating drug abuse in the United States readily approved the proposal, officials said.

Bush heads the National Narcotics Border Interdiction System, the government's coordinating effort for fighting the drug trade, which grew out of a smaller effort in South Florida in 1982.

Officials familiar with the Reagan directive said it includes a series of findings that criminal drug trafficking can corrupt political and economic institutions in the United States and other nations.

It also concludes that some insurgent and terrorist groups are cooperating closely with drug traffickers and use this as a major source of funds to carry out their activities. The directive then makes it U.S. policy to try and help other nations "confront and defeat" the drug traffickers and terrorists, officials said.

Among the actions authorized by the directive, officials said, are expanded U.S. military support for such operations, greater participation by the U.S. intelligence community, improvements in the use of electronic eavesdropping, more assistance to other nations in establishing their own drug abuse control programs, and additional emphasis on drug trafficking in discussions with other countries.