For decades, Arlington had been a relatively homogeneous bedroom suburb whose cosmopolitan claims were limited to a few French, Italian and Mexican restaurants and whose big-city life atmosphere came strictly from the skyscrapers of Rosslyn.

But politics across the Potomac and political turmoil in nations thousands of miles away have changed all that in the past 10 years.

As the county's population dropped by nearly 23,000--from 175,000 residents in 1970 to 152,000 today--the percentage of foreign-born residents jumped dramatically. Arlington's Hispanic community grew by 40 percent and its Asian community by nearly 250 percent in the last 10 years, census figures show.

Today, an estimated 15 to 20 percent of county residents are foreign-born, many of them refugees and illegal aliens who speak little or no English.

Since Saigon fell in 1975, more than 8,000 Indochinese refugees, most of them Vietnamese, have migrated to Arlington and sought help from the county, according to Martin P. Wasserman, director of the county's Department of Human Resources.

That gives the county the highest per capita concentration of Indochinese in the nation, he says.

But the foreign-born population is by no means entirely Indochinese. Gloria Bustillo of the Spanish-Speaking Committee of Arlington estimates there are about 10,000 Hispanics in the county. Foreign-born county residents also have come from such countries as India, Pakistan, Afghanistan, Turkey, Greece, Hungary, Ethiopia, Iran, Poland and the Soviet Union.

Arlington's facelift was not predetermined, however. The influx just happened.

It's been "like a Catch-22 situation," said Stephen H. Detwiler, county board chairman. The more the county spent to help the refugees, the more Arlington's reputation spread, attracting new waves of immigrants.

Their numbers are expected to continue to grow in the next few years, but not by the significant leaps of the past decade.

"We've gone through that radical shift of not having an identifiable refugee population to having a significant one," said Detwiler. "The transformation has occurred; it's behind us now."

What's ahead is the adjustment phase: learning to adapt to this vastly changed, multicultural community.

The dramatic changes are especially marked in the schools. Today, in 10 of the county's 33 schools, minorities, including blacks, account for more than half the enrollment. Though there are Asians and Hispanics at all schools, their enrollments combined run from 30 to 60 percent at those 10 schools.

School board member Torill B. Floyd recently called for a school staff study of the effect the trend is having on the education process. Parents recently have begun complaining that their native-born, English-speaking children are getting less attention because teachers have to spend more time with students who are not fluent.

In kindergarten through 12th grade, nearly 12 percent of the 14,761 pupils need extra English instruction, up from 8 percent two years ago; an estimated 30 percent of the 23,000 students in various adult education programs have limited or no knowledge of English.

"It's an emergency situation right now and it needs immediate attention," Floyd said.

But the impact also has been severe on the county's health and welfare agency, the Department of Human Resources.

Nearly 40 cents of every dollar spent on public health needs in the county now go to refugee needs, said Wasserman. About 10 percent of that is spent on curbing a tuberculosis rate that has skyrocketed among Indochinese refugees, he said.

In addition, the refugee welfare caseload nearly tripled in the past two years--from 490 in January 1980 to 1,380 in December 1981.

Wasserman said 90 percent of the first wave of refugees, those who arrived between 1975 and 1978, got off public assistance rolls after their first 18 months. But the refugees entering the country today lack the education and skills to get off relief that quickly, he said, and are more likely prospects for the regular welfare rolls.

Faced with pending cuts in federal, state and local funding, Wasserman said he is trying to cut back on nonvital services to avoid hurting either the indigenous or the refugee populations. But, he warned, "the potential for public backlash is there" if too many services for native-born residents are curtailed to divert funds to refugee programs.

Throughout the county, officials are sensitive to this potential for backlash, but they contend there is no basis for alarm.

"You always have to be concerned about that possibility," said school Superintendent Charles E. Nunley. "But I don't think we've been neglecting anyone. We're doing a pretty good job with all our students on achievement."

Added Detwiler: "What we have to make sure of is that our existing population is served adequately, particularly in the Department of Human Resources; that (services) are not taken from them to beef up an assimilation program.

"You don't control an influx directly, except maybe by denying existing services. But the whole population would suffer then, and you don't want that."

A "master plan" for dealing with the financial and cultural impact of Arlington's growing foreign-born population might be "worthwhile," Detwiler said, but "the difficulty with planning is you don't know what to plan for; there are no distinctive figures to work with."

Wasserman estimates that Indochinese refugees continue to enter the county at an average rate of 100 a month, based on the number who seek help. But no one seems to know how many refugees--from Southeast Asia or elsewhere--come to live in Arlington but don't ask for aid or how many illegal aliens are settling in the county. The county has no office or person responsible for refugee affairs.

The county does know how much it's spending, however. The county government spent an estimated $4.4 million this fiscal year alone on refugees, including $3.8 million given the schools for educating refugees, according to the county fiscal affairs office. The federal and state governments contributed an additional $4.6 million for refugee aid in Arlington.

"We have gotten far more than our fair share (of refugees)," said county board member Walter L. Frankland. "And we definitely need our fair share of help from the federal and state governments." Frankland and board member Dorothy T. Grotos are especially outspoken critics of the federal government's failure to help finance a larger chunk of the resettlement costs.

For refugees who call Arlington their first American home, the county bears the responsibility of assimilating them into a culture new and strange to them. It is the first place some of the newcomers receive regular health care and the first place they get a chance to learn English and the job skills needed to get them off the welfare rolls.

But, Wasserman lamented, "After we've given them all that initial training and investment at the most costly time (of their adjustment), they will ultimately settle outside when they can't find (adequate) housing here and their taxes will go someplace else."

Again, no one seems to know just how many refugees pass through the county on their way to settling elsewhere, but the feeling is that the number is great.

All five county board members say they get only a handful of calls from native-born residents complaining that the county is focusing too much attention or money on the refugee population. And they get even fewer calls from the refugees themselves.

"In political terms, they are not an identifiable constituency, although that may come in time as they establish themselves and get citizenship," said board member John G. Milliken. "They seem to work pretty well within their own networks and organizations, and they seem to know who to contact at the staff levels if they have a problem with housing or something that's health-related."

"There's a strong feeling they'll be able to establish themselves," he said, without excessive help from the county.

"Their only impact is a favorable one," said Larry Blackwood, president of the Ballston-Clarendon Civic Association, an area where many Southeast Asians have opened small businesses. "They're the only people keeping the Clarendon business section alive today."

The Indochinese have been welcomed warmly in Arlington as hard-working, exceptionally polite people with a great respect for authority. They accounted for less than 1 percent of arrests made last year, and police say they are more often the victims than the perpetrators of crimes.

They also generally have been accepted by the black community, which represents 9 percent of the county's population. There has been some resentment from blacks, however, said John Robinson, who directs the Martin Luther King Community Center in the predominantly black Green Valley neighborhood.

"I've seen kids call them names because they're not used to seeing foreigners in Green Valley," Robinson said. "And some people don't like (the fact that) they're getting jobs blacks used to take . . . or that (the center) gave some of them some clothes at Christmas time.

"But they're very nice people, very humble people--and since they're here, we've got to treat them like human beings."

Robinson maintains, though, that the federal government should have done a better job with refugee orientation. He says they've been left ill-prepared to cope with many aspects of life here.

It is not uncommon on arctic mornings when school opens a few hours late, for example, for a maintenance man to find a shivering huddle of Indochinese students standing in front of a school at the normal opening hour. Their parents, explained a school spokeswoman, probably do not understand the English-language radio announcements about delayed openings and cancellations.

In fact, much of the orientation burden has fallen on the schools.

At many schools, principals have produced bilingual "excuse notes" for students who have to miss all or part of a day. Parents can check the appropriate box by an explanation in their native language for such things as "doctor's appointment" or "religious holiday," and the school secretary can read the English translation alongside.

English is not the native language of nearly 20 percent of Arlington's public school students. Though not all need additional help, the increasing number of those who do has prompted parents to complain to the school board that their native-born children are getting less attention from teachers.

As a result, the school administration is studying whether there should be more intensive English instruction in the lower grades, more grouping by proficiency or even busing of students to ease lopsided enrollments of non-English speakers in some schools.

Deciding how to address the increasing enrollment of non-English speakers "is going to be one of the biggest issues we face in the coming year," said school board member Michael E. Brunner. "It's a tough issue, and there's going to be no easy way to deal with it."

Chairman Claude M. Hilton, however, disagrees. "Obviously you have to bring the kids along, help them learn English, and you may have to staff (teachers) differently. Other than that, there really isn't a problem; and I believe, I hope, they perceive things are going well."

School officials do agree on the need for more teachers and aides to work with foreign-born students. Fearing federal cuts in the program that pays for these special teachers, the superintendent proposed a budget for next year that includes money to hire six new teachers and three new aides.

"Our future is here," Tri Khac Pham, president of the Vietnamese Parents Association, recently told the school board. "Please (continue to be) patient and let us have more time for cultural adjustment.

"We hope one day we can do something for the prosperity of Arlington County, which has accepted us with open arms and generosity."