The Defense Department must reduce the number of American military dependents now in Europe to avoid "the pandemonium" that would occur if war broke out there, Sen. John C. Stennis (D-Miss.) said yesterday.

Stennis, who is chairman of the Senate armed services committee and the Appropriations subcommittee of defense, is a leader of a group in Congress that has collided head-on with Pentagon civilian leaders in a new drive to cut the number of wives and children living with servicemen in Germany.

"We've got to phase it down to manageable proportions," said Stennis of the contingent of dependents during an interview.

"It is intolerable" to have so many dependents in Germany that they would interfere with military movements during wartime, he said.

Pentagon civilians counter that the promise of a glamorous overseas assignment, with family, is an essential lure for attracting volunteers into today's military. But many military commanders in Europe feel that West Germany is already overloaded with dependents, many of them disillusioned becaused they cannot find decent housing or afford the high cost of living.

Gen. Alexander M. Haig, commander of allied forces in Europe, told a Senate Armed Services subcommittee last week that sending more dependents to Europe-as the Pentagon now plans to do-would make a bad situation worse.

The Pentagon estimates that 371,400 spouses and children of service peoples live overseas today, and 294,000 of the total are in Europe, mainly in West Germany. In 1969, 228,425 dependents were in Western Europe-a period when military pay was considerably lower and many dependents had to pay their own way overseas.

"This thing has just grown and grown," Stennis complained.

Starting last October, the Pentagon made it easier for dependents to go overseas by agreeing to pay the travel and moving expenses of the dependents of every serviceman from the rank of private up rather than just senior ranks as was the case previously. Almost half of today's enlisted force is married compard to only one-third in 1954.

Fearing that the additional travel money would further overload overseas bases with dependents, Congress last year tried to reverse the upward trend by limiting the Pentagon to a total of 350,000 overseas dependents. But, after some sloppy arithmetic, the services found that they had more than 350,000 dependents overseas at the time the law was passed.

Robert B. Pirie, assistant secretary of defense for manpower, has suggested to the Senate that one way around the ceiling problem would be to count only those dependents who has gone overseas at government expense, not those who got there on their own. But many European commanders contend the argument over arithmetic misses the point.

Life is so miserable for thousands of dependents, these commanders are telling Congress, that the Pentagon should scrap the idea of sending even more of them to overloaded places like Germany and should send soldiers without their families for 18 months-rather than the usual three years. (Soldiers were sent to Vietnam for 12 months without their families.)

But Pentagon civilians fear this move would lower the morate of the troops and cost additional millions of dollars in moving expenses.

The military dependents are stepping up thei correspondence to congressional offices to spotlight the difficulties of living in West Germany these days on an enlisted serviceman's pay. Wrote one Army wife to the Senate Armed Services Committee: "I wonder if anyone has considered the traffic jams that would precede" an attack on Germany as dependents rushed to get to the airport. Even without a war, she continued, "the cost of living (in Germany) is tremendous . . . housing is another problem. We have people arriving here everyday who can't even get into the Army hotel, much less housing . . ."

"The snack bar is so overcrowded that it is impossible to get a hamburger at lunch in less than 45 minutes; post exchanges and clubs are also crowded beyond being usable.

You have to schedule dental appointments, eye examinations and doctor's appointments months in advance" because medical facilities are overextended.

"Defense Department schools are a joke," continued the leter. "Discipline is nonexistent, drugs are many times easier for children to buy."

Stennis said that his committee has not decided how many dependents would be considered manageable but will insist tht the Pentagon develop a program addressing the problems of overseas dependents.