FROM MANAGUA the Ortega brothers have been reporting on the shape of military things to come. First, Humberto, Sandinista defense minister, said that Nicaragua was halfway to building, with Soviet support, a 600,000-man defense force and that the country had ''not renounced'' acquisition of advanced weapons -- including MiG fighters of a sort Washington insists it won't countenance. The next day, Daniel, Sandinista president, said that the defense plans presented by his brother were merely a military ''proposal'' to the government and that in peace Nicaragua would have only a ''modest army.''

Humberto Ortega, it is suggested, was either sounding off defiantly in the way Sandinistas do or putting an arm on the Kremlin, whose spokesman, in Washington, ducked the whole question of what military aid Moscow might yet provide. But he was also raising the specter of an immensely overarmed state on the make that gives the rest of the region nightmares. Presumably that explains why Daniel Ortega jumped in to assert that his country was ready to negotiate arms reductions all around. Nicaragua, he said, could not afford a huge standing army, although it wants to have the entire population trained to defend the country -- in the manner of the Swiss.

The difference between the Ortegas may be less than meets the eye. Humberto expresses the Sandinistas' strategic ambition, Daniel their tactical discretion. Which line will guide Nicaraguan policy? It would be the height of foolishness, especially on the part of other Latin Americans, to disregard the ominous shadow cast by Humberto Ortega. The moderation suggested by Daniel Ortega remains at this point entirely air.

Enter Mikhail Gorbachev, who is in a position to make it come out either way. Nicaragua will be a prime place to test the sanguine forecast that he is now turning down the heat in the Third World. From what is known, Nicaragua was discussed for all of 15 seconds at the summit. Reportedly, the Soviet leader offered to suspend military aid to Managua if the United States cuts off the contras in the context of the Central America peace plan. The American government says it intends to follow up. Meanwhile, the Ortegas' remarks will enter the American debate over contra aid and -- more important -- the Central American decision due next month on whether Nicaragua is complying with its peace pledges. What will Managua do in the next few weeks to convince its neighbors it is not planning to overwhelm and terrorize them?