BONN -- The triumphant visit to West Germany of Erich Honecker, the Soviet puppet ruler of communist East Germany, strongly suggests that the shadow of influence cast by Soviet leader Mikhail Gorbachev is lengthening over the politics of this foremost U.S. ally in Europe.

Denied passage here on at least two previous occasions because the timing did not suit Moscow, Honecker has now proved his point with his two-day visit to the West German capital. It is this: that East Germany must be accepted by its far more powerful sister state as a sovereign nation. For decades, the myth that this could never be was a fierce tenet of the Christian Democratic Union, which Chancellor Helmut Kohl now leads.

The disappearance of that myth into the cool breeze off the Rhine that fluttered side-by-side flags of East and West Germany this week must be counted a political triumph for Gorbachev. It dramatizes the appeal of his new de'tente, not only here but for President Reagan and the Democratic-controlled Congress. As Washington responds to Moscow, so does Bonn respond in dealing with the slippery politics of Central Europe. Honecker's visit may be only a beginning.

It broke no new ground in terms of eventual German reunification. In fact, hardly anything happened on the surface during 12 hours of Kohl-Honecker talks, four of them with no one else present. When it came time to draw up a communique', there was no haggling. Considering the objectives agreed upon as a condition of the visit, there was nothing to haggle about.

That was fine for the small, trim 75-year-old Honecker, who looked almost frail compared with the huge and hearty chancellor. He had made his point simply by sharing the world stage with Kohl.

As for Kohl, buffeted for years along with previous chancellors by damaging shifts in U.S. policy, the visit was a political ten-strike of a different kind. It enabled him to grab a big piece of the new political pie of de'tente that usually goes to the opposition Social Democratic Party and to his Free Democratic coalition partner, headed by Foreign Minister Hans-Dietrich Genscher.

Genscher's pint-sized FDP gained votes at Kohl's expense in last January's election. One reason: Genscher's canny play for de'tente, which became wildly popular when Gorbachev pulled it out of mothballs and Reagan endowed it after six years of tough talk. So long as the Reagan-Gorbachev de'tente thrives, just so long will Kohl's or any other government here match or exceed Reagan's pace.

That raises serious questions for the future. One Western diplomat called Honecker's reception by Kohl a ''tremendous admission'' of failure by the Western alliance, with Kohl simply doing what he has to do -- accept the new reality. But danger signs line the near horizon. Sentiment in this front-line NATO state is shifting in waves toward a conviction that the Soviet threat really is diminishing. Polls show that Gorbachev, not Reagan, is getting the political windfall from de'tente.

Significant figures in the SPD now complain openly that the army is costing too much and should be stripped down to a kind of national guard. That is talk guaranteed to trigger congressional demands for a reduction in U.S. forces here.

When we asked a key policy maker if voters would vote for or against the total withdrawal of all foreign forces from both Germanys, he paused, then finally said: ''I think we would still find a majority'' against the proposition. Ten years ago the vote would have been solidly against, showing an ominous trend line.

Despite all his post-Iran problems, Reagan alone has the power to avoid more policy shifts such as the shocking Reykjavik offer of total U.S.-Soviet denuclearization made last fall. Even without such errors and their damaging effect on U.S. credibility, Reagan is learning from Bonn that managing the new de'tente in competition with Mikhail Gorbachev is a challenging and, so far, losing proposition.