BONN -- Although the full impact of President Reagan's rush to de'tente still lies ahead, its risks were exposed here when the leading Social Democratic strategist on East-West relations told parliament that ''security by military strength'' is an ''illusion.''

Those words in last week's defense debate by Egon Bahr, long the key Social Democratic Party leader, angered Defense Minister Manfred Woerner, a stalwart of the Christian Democratic Union. He leapt to his feet in the Bundestag and accused Bahr of proposing ''nothing more than hope or wishful thinking.''

But Bahr is riding the flood tide of the Mikhail Gorbachev-Ronald Reagan courtship and nuclear disarmament. The CDU's long dedication to military strength as the true path to security is suddenly made to look out of date. In Bahr's surprising words, ''Military confrontation will be replaced by a peaceful contest of systems, ideologies and economic cooperation.''

Chancellor Helmut Kohl, the CDU leader, said privately that his decision to scrap West Germany's own 72 Pershing 1 missiles was rendered inevitable by the political impact of East-West de'tente on German politics. Most defense-oriented CDU policy makers agree. ''We were made to feel,'' one CDU defense strategist in the Bundestag told us, ''that the garbage was already in the bag.'' He meant that the U.S. promise to modernize the German-owned Pershings was irrelevant because Kohl knew the Bundestag would never approve modernization -- with or without U.S. backing.

The slow rot that some German leaders feel is starting to destroy the NATO consensus on how to preserve European security has now moved from the Pershings to the next level down: ''short-range'' missiles up to about 500 miles.

Kohl has signaled a desire to get rid of short-range weapons because if they are ever used, it will only be on the soil of East and West Germany, letting the rest of Europe escape nuclear devastation. But the United States wants to modernize these short-range weapons, particularly the 250-mile Lance missile. Thus, the ''double-zero-zero'' -- the elimination of all U.S. and Soviet missiles down to 500 miles -- has already opened a breach between Washington and Bonn on the next stage.

The loss of trust in the United States has not been so great since Jimmy Carter suddenly canceled the neutron bomb after months of political pressure on his allies to approve its deployment in Europe. The basic concern today is that President Reagan rushes toward massive elimination of nuclear weapons in Europe without assurance that an equally safe deterrent is ready to take the place of the nuclear deterrent.

That fear is compounded by an inexplicable (and so far unreported) top-secret briefing for key Bundestag members in Washington this summer, apparently given by the CIA. The thrust of the briefing, according to both CDU and SPD political leaders who received it is this: the United States now believes that Soviet ability to launch a surprise conventional attack on NATO forces poses a much smaller risk than NATO doctrine has preached for 20 years. That means the call-up of Soviet reserves would give the West plenty of warning time if a surprise attack were ever launched.

We were told by one CDU leader who received the briefing: ''The newest CIA analysis claims that Soviet conventional forces are deployed for defense, not offense, which, if true, is a most surprising change in the threat perception held by NATO all these years.'' The suspicion here is that the new ''threat perception'' may be politically inspired to reduce fears of defenselessness against conventional Soviet attack once Euro-nuclear weapons are gone.

American officials in Bonn claim that the briefing reflected only one of many opinions in the intelligence community and that German officials who received it are making far too much of it. Nevertheless, Bahr and other left-wing leaders claim it justifies their attacks on the German defense budget; right-wing leaders are using it to justify their growing disillusion with Reagan's arms control policies and their distrust of the United States.

So rancid a mood clearly poses a danger of destabilizing the Western alliance, but the Reagan administration's beady-eyed pursuit of the new de'tente may be blinding it to that danger.