Army Spec. 4 Milan Paich was driving a truckload of Hawk anti-aircraft missiles down from a hilltop position during a maneuver recently when a soldier directing traffic waved him into the path of an oncoming tank. Paich stopped, but the soldier kept waving and screaming at him to move and when Paich got close up he knew why.

"The guy wasn't right," he says, meaning he was on drugs.

The mishap was avoided and the incident passed unnoticed. Yet, it is the kind of thing that casts a shadow of uncertain length on the readiness of the 200,000-man U.S. 7th Army in Europe - an army that in many other ways has made enormous progress in its combat capability since the Vietnam war ravaged its spirit and the Middle East war of 1973 dented its stockpiles.

A young soldier in the mess hall here, with a newsman and a senior officer sitting right next to him, says matter-of-factly that perhaps 75 percent of the men in his unit, in his opinion, are taking either too much drugs, too much alcohol or both."

The soldier is stationed at a remote air-defense site, where troops traditionally have less to keep them busy. Many other soldiers say such figures are gross exaggerations.

But clearly the Army still has a drug problem.

"The best answer I can give you, says Lt. Gen. David E. Ott, commander of the 7th Corps, the largest unit within the 7th Army, "is that we don't really know ourselves the full extent of the problem. We all sense the frustration and try to determine the true magnitude. But the way I must put it into perspective is to ask if it exists to the extent of reducing combat effectiveness of my corps. "The only answer is that I can't find where it does. Yes, there is drug abuse. Most appears to be during off-duty hours, like beer drinking. The small number who become addicts we usually find and eliminate fairly soon," he says. "But I have never," the general streses, "seen any unit in this corps even slightly degraded in its ability to go on alert, take a test, move out on an exercise or anything else."

What about a Saturday night alert? He is asked. "We would have a combat-ready force assembled within two hours," he replies.

Interviews with dozens of senior enlisted men and officers throughout Germany - with a broad view of a unit's ability to perform in the field - tend to confirm Otts assessment.

The drug and alcohol abuse problem seems to exist as something of a phantom. Experienced soldiers say the Army can move unhampered when it must. Yet virtually every drug in use, even the hashish widely used in Germany, distorts the senses for at least two hours, according to Lt. Robert Dumas, who operates a community and drug control center at Bad Kreuznach.

Another problem in assessing the extent of drug usage is that more soldiers talk about it than actually use it or know for certain who is using it. Several soldiers, pressed on this point, acknowledged that they didn't really know anyone personally using heroin.

At Kaiserslautern, a rear area where drug use is normally more widespread, Spec. 4 Bobby Dawson said that in two years he had "never seen anybody who was really spaced out and couldn't do his job."

Officially, the Army's European command acknowledges that drug abuse has resumed an upward swing this year after dipping in the mid-1970s from the Vietnam era rate.

Last yeat 40 GIs died in Europe from drugs and the rate this year is even higher, with 25 deaths recorded in the first six months.

Latest surveys, based on urine testing and questionnaires indicate that 19 percent of the troops abuse cannabis, which includes Hashish and marijuana, and 7.8 percent of the troops abuse dangerous drugs such as heroin. The problem, however, is that hashish unless it is cut with heroin - doesn't show up in urine analysis, so the extent of use could be wider than the questionaires indicate.

Beyond drugs is an alcohol problem which, according to Sergeant-Major Terry Trobough, a 22-year veteran with the 2nd Armoured Cavalry Regiment, seems worse these days than the drug problem.

"A guy on heroin is of no use. He is a danger to the unit and we get him out fast. Alcohol is tougher to deal with," says Sgt. Charles Smith of the 1st Armoured Division.

In the last year, a 1,300-man squadron of the 1st Armoured Division here has had roughly 100 men out of action and in drug or alcohol rehabilition programs, officers report.

At Bad Krueznach, headquarters of the 8th Infantry Division, Dumas and Capt. Samuel Barnes, the drug and alcohol abuse officer for a military community of about 2,500 persons, report they are treating about 125 RPT 125 soldiers, split about evenly between heroin and alcohol problems. In the past 10 months, 26 new heroin cases were found in tests of about 400-500 soldiers.

At a minimum, heroin will distort a soldiers perceptions for four hours. Addiction can drive him into a three-bag-a-day habit. The current rate is about $20 a bag.

"That's a $21,000-a-year habit that leads to crime, extortion and even pushing," says Barnes.

Though Army crackdowns have largely kept heavy drug peddling and intimidation out of the barracks, some so-called "enforcers" still hold sway over small groups.

When a spot urine check turns up a soldier with traces of heroin in the sample, he is referred to the drug control center. He is not punished unless he is a pusher or has fresh heroin in his possession. He suffers no loss in pay. The visit is recorded only in his private medical records and not in his enlisted service record, and he usually is back in his unit the same day. Dumas says. The idea is to encourage soldiers to seek help without fear of punishment.

Repeated offenders, however, could wind up with Chapter Nine Discharges, which are honourable but which indicate an administrative discharge for drug or alcohol failure, as the Army calls it. Soldiers judged dependent go through detoxification programs and about 90 percent return to their units. Those still not cured are sent to Veterans Administration hospitals and then out.

The alcohol problem is older and more subtle.

"Barracks life can be tough," says Sergeant-Major Clarence Fasan in Kaiserslatern, "and where do kids go to get away? Into town or to the enlisted men's club to get a drink. A lot of things are unthinkingly associated with drinks."

To help curb this, the Army is ordering and end to two ancient traditions: No more "happy hours" of cheap drinks at service bars and no more "wetting down" parties for promotions