President Carter's humiliating treatment of Chancellor Helmut Schmidt in the neutron-warhead affair has weakened West German self-confidence to handle another issue of larger importance under U.S. "leadership."

That issue is Moscow's sudden acceptance of "parity" in the long-stalled effort to reduce Western and communist troops in Central Europe.

This means that the implications of Carter's mismanagement of the neutron affair are far from exhausted. To a degree that unseeing eyes in Washington seem unable to comprehend, the humiliation of the Bonn government has subtly undermined Schmidt's ability to deal from strength against other Soviet political operations.

The first of these is the troop-reduction issue. Fresh from his triumph in killing off the neutron warhead, Soviet President Leonid Brezhnev unexpectedly did two things: He came here for a highly visible state visit, his first in five years (giving the chancellor's office just three weeks to prepare for it); and he presented to the world his lovely little package of "mutual and balanced force reductions." For the first time, he accepted the West's demand for equal ceilings - parity - for NATO and the Warsaw Pact.

In Washington, reaction was swift in approving the Soviet acceptance of parity - 700,000 on each side - as a major breakthrough. That sets the stage for a predictable new Soviet propaganda offensive in the name of peace. But there are catches. Catch One is Moscow's new insistence that parity already exists in Central Europe, meaning all that is needed are equal reductions on each side to 700,000.

In fact, the West knows that communist forces are now around 175,000 larger than NATO's. A struggle over this fundamental disagreement on what the experts call the "data base" is now ensured, with the truth difficult to prove.

Catch Two in the Soviet proposal is that it is aimed straight at West Germany. It ignores the West's demand that the 700,000 ceiling be reached by "collective" reductions, not by allocations to individual states. Moscow's proposal as read here would dump "collectivity," demanding country-by-country ceilings. That underlines cardinal Soviet postwar strategy: to systematically reduce West German power and influence.

As of today, there is no question that the Carter administration will insist both on true parity and on collectivity, in lock-step with West Germany and other NATO countries.

But there is a queasy feeling here about the coming Soviet propaganda campaign for its plan. "They will have public opinion on their side," a leading strategist in Schmidt's government told us. "It is going to be difficult to make the Soviet proposal look as bad as it really is."

And then there is Carter's neutron example. The top-level West German Security Council, egged on by the Americans, secretly agreed in January to support U.S. construction of the neutron warhead, and President Carter was personally informed of this government decision. That followed six months of comprehensive conversations with the Carter administration on the weapon's military, political and strategic implications. There was no disagreement between Bonn and Washington.

Schmidt's understanding was explicit: The weapon would go into production; for the ensuing two years the Russians would be pressed for concessions (removal of the SS20 mobile missile or reduction fo the huge Soviet-European tank force) as a quid pro quo against actual deployment of the neutron; if no deal could be struck, the neutron would be deployed.

To carry this issue against his muscular left wing, Schmidt fought a bitter and debilitating rear-guard action from the summer of 1977 to the day in March that Deputy Secretary of State Warren Christopher flew to the chancellor's home in Hamburg to tell him Jimmy Carter had changed his mind. (Stunned, Schmidt asked in effect: Are you speaking with your master's voice? His and mine, said Christopher.)

"Carter made Schmidt look like a jackass," one Western diplomat told us.

But beyond that forgettable personal affront, the neutron-warhead affair undercut the Germans throughout Europe, leaving the Soviets as victors. The capitulation of Jimmy Carter proclaimed the rectitude of Soviet propaganda.

To consolidate his victory, the important troop-reduction issue was immediately opened up by Brezhnev as the next order of business - but not before Brezhnev made his splashy, long-delayed visit to Bonn to show the boys here what a winner looks like.