A season of decision is coming for the United States with respect to Ethiopia, the Marxist run African state where the Reagan administration has been conducting a policy so pragmatic as to pain many true believers. The policy is one of providing massive food aid and of withholding support from the guerrillas fighting the central government, even while doing stern political battle against that government. Critics see this as helping Moscow sustain a client and build socialism. With the ending of the need for massive food shipments, there is new pressure in and on the administration to add Ethiopia to the list of states where the United States supports an anticommunist insurgency.

Reagan policy has bet that by leading an immense international relief campaign, the United States not only could save millions of lives but also could offset much of the influence the Kremlin was gaining by arming Marxist dictator Mengistu Haile- Mariam against secessionists. The food aid has produced what a Senate report calls "extraordinary pro-American sentiment" among Ethiopians; finally, officials publicly acknowledge the American contribution. Still, Col. Mengistu has valued more the Soviet arms that have let him impose his power and keep Ethiopia territorially intact.

This is the context in which a food aid cutoff and arms for insurgents are being weighed. The rationale for stopping relief is that Ethiopia is passing into a new post-drought stage of agriculture and politics in which the government's Soviet-style forced collectivization policy is the chief threat to a secure food supply. "Villagization" could affect millions; by contrast, the government's coercive resettlement of peasants from secessionist areas uprooted "only" 600,000 people. Both programs are said to be in temporary suspension. Not ideological chauvinism but considerations of humanitarianism and efficiency require all those who kept Ethiopia alive in the drought to demand now that it go the productive market way.

The administration's liberation rhetoric is regularly cited by those who want to arm insurgents. But the official policy has been restrained. The principal insurgent groups are not only Marxists but secessionists -- twin disqualifications. A recent story in this newspaper disclosed that the CIA had been in touch with some anticommunist dissidents; in the course of a propaganda operation two years ago, a CIA man was apparently caught and tortured before being released. Planning for moving up to a covert guerrilla operation is now said to be in a preliminary stage.

But others -- mostly Moslems -- already support the guerrillas. American policy ought to keep underlining the advantages to Ethiopia of easing political strains and opening to the West.