The search for good reasons for rejoicing about the U.S.-Soviet agreement on intermediate-range nuclear forces is like the 19th-century search for the source of the Nile: difficult. It is, however, unlike that 19th-century search because explorers knew the Nile had to have a source.

The agreement will retire less than 4 percent of the worldwide inventory of warheads. The retirements are concentrated in Europe and will magnify the importance of the enormous Soviet conventional-force advantage there.

It is preposterous to say that domestic exigencies drove the Kremlin in this direction. The Kremlin did not seek this agreement to achieve economic respite. The agreement will have no measurable effect on either side's defense spending.

The agreement removes from Europe the weapons the Soviets fear most, the Pershings. They can penetrate the Soviet Union with certainty and can strike targets accurately in 13 minutes. The Soviets will dismantle no weapons that could strike America. All the American weapons that will be dismantled could strike Soviet territory.

Some defenders of the agreement say the weapons to be scrapped were too vulnerable to be valuable. These defenders have a point.

There are two ways to reduce the vulnerability of weapons -- armor or mobility. Missiles are ''soft'' weapons; a bullet can pierce them. The Pershings and cruise missiles were supposed to be quickly dispersible during crises. But host countries have put unreasonable restraints on the movements of the weapons (which, because of Pentagon maladministration, have cumbersome logistical accessories). In a crisis, U.S. requests to host countries to disperse the missiles probably would be rejected as provocative.

Restraints on the movements of the weapons reflect Europe's haunted memory. The specter in America's historical memory is Pearl Harbor, a lightning strike. Europe's obsession is, understandably, the First World War, which began with mobilizations that became inexorable. Pershing and cruise missiles, immobilized by Europe's historical anxieties, could be destroyed at their bases by Soviet conventional weapons.

The administration hails the INF agreement, as all agreements are hailed, as a ''first step'' toward grander things. But wiser heads hope it is a last step, a prophylactic measure to enable the United States to declare an end to arms control in Europe. They hope it will anesthetize Europe's peace movement and America's arms-control clerisy. It is supposed to be arms control to end arms control. It is supposed to ease the pressure on Western parliamentarians who are under constant pressure to cut defense budgets in order to enrich welfare-state benefits.

But arms-control agreements whet the thirsts they are supposed to slake. The INF agreement will energize the forces pushing for denuclearization of Europe. President Reagan's recent rhetoric has contributed to the stigmatization of nuclear weapons.

The INF agreement will require the Soviets to destroy many more warheads than the United States. But that asymmetry is a small price for the Soviets to pay for the consequent enhancement of conventional forces. To enhance European stability now would require Soviet consent to another asymmetrical reduction, this time of conventional forces. They have no incentive for that.

Military historian John Keegan says the agreement may recover for the Soviet Union much that it lost in Europe through two postwar blunders. In 1945, the Soviet Union withdrew from Yugoslavia, thereby forfeiting access to the Mediterranean. After the 1956 Austrian treaty, it withdrew forces from Hungary, thereby igniting the revolt that cost communism its ideological e'lan. But the constant Soviet aim -- military dominance of the continent -- requires neither Mediterranean access nor ideological e'lan.

Soviet military dominance is advanced by agreements that reduce nuclear forces without reducing conventional forces. The INF agreement rests on the fallacy that any subtraction from nuclear inventories makes the world safer.

The Soviet approach to arms control has nothing to do with ''control'' as the Western public understands it -- nothing to do with reducing the importance of weapons in the relations between nations. Rather, the Soviets use arms control to impede the West's procurements and deployments, to channel arms competition in directions disadvantageous to the West and to produce de'tente, the climate conducive to Soviet parasitism -- the theft and subsidized purchase of Western technology.

The closest one can come to good reasons for accepting the INF agreement is this: arms agreements are inevitable. Democracies demand them. This is minimalist arms control; it is, strictly speaking, the least we can do. And perhaps the recrudescence of de'tente can be minimized.