In an eerie collision between make-believe and reality, the staff at Western alliance headquarters here was engrossed last week in a periodic war game that this year dealt with a military crisis in the Gulf.
While American and British naval forces were actually steaming to the troubled region in anticipation of a possible escalation of the Iran-Iraq war, NATO planners were engaged in a roughly analogous exercise.
Code-named ''Helix 84,'' the drill was designed to cope with a fictitious Soviet military incursion into Iran, accompanied by a buildup of Warsaw Pact forces on the Yugoslav border. In response, the NATO machinery was going through the motions of dispatching the US Rapid Deployment Force to the area along with some US support from Europe and replacing the latter with European troops and equipment.
A Western source at NATO headquarters commented that the highly secret exercise was in no way connected with any real activity in the Gulf.
The operation is also said to involve large numbers of personnel not only at NATO headquarters but also in the capitals of member governments. It is designed to test effectiveness of the bureaucracies to deal with different types of crises.
While NATO military exercises are generally confined to a specific area, usually inside NATO territory, the source noted, the civilian exercise can be more imaginative and dramatic.
But it was also underlined that these biennial operations are usually highly classified and NATO leaders were said to have been disturbed because fragmentary reports filtered into the Western press this year and were instantly seized upon by Pravda and other Soviet news media as examples of Western bellicose intentions in the tense Gulf region.
The actual military operations there by US and British forces are not NATO-connected and are precautions against the possible interruption of oil tanker traffic in the Gulf because of the escalation of the Iran-Iraq war.