NEW YORK, June 20 (Reuters) - Go east, young lawyer. Or west. Or anywhere there's legal work outside the United States borders.

As the economy continues to struggle, the marching orders for some of the biggest U.S. law firms is to grow abroad. In recent months, Davis Polk & Wardwell opened an office in Sao Paulo; K&L Gates and Cadwalader Wickersham & Taft in Brussels; King & Spalding in Moscow, and McDermott Will & Emery in Paris.

The expansion is driven largely by energy work, foreign regulatory matters, finance and transactional practices. It reflects both the financial strength of top-tier firms, and the belief that the opportunities for domestic growth down the road are limited.

Some of this is playing catch-up with foreign competitors. Compared to U.K. law firms, for example, most U.S. law partnerships receive a smaller percentage of their revenues from overseas work-less than 25 percent, versus more than 50 percent for U.K. firms, according to Tony Williams of Jomati Consultants in London.

Moreover, there is anecdotal evidence that firms with a large international practices may be better able to weather economic downturns. At its peak, only about 17 percent of the lawyers at now-defunct Howrey were in foreign offices. Altogether, failed firms Thelen, Powell Goldstein, Thacher Proffitt & Wood and WolfBlock had just 30 attorneys in foreign offices among all of those firms combined in 2007, according to data from The National Law Journal.

Having shaved expenses and shed attorneys during the recession, revenue per lawyer among the top 100 U.S. law firms in 2010 was up 4.4 percent, according to The American Lawyer. The increase followed a 2 percent decrease in revenue in 2009. Yet the firms are also proceeding with caution, chastened by the recent collapse of law firms like Howrey, Thelen and one-time international powerhouse Coudert in 2006. "There's not a gold-rush mentality," said King & Spalding partner Mark Thompson, whose firm's new Moscow office will focus on energy and transactional work.

One of the most aggressive players in the field has been DLA Piper, which is now in 30 countries. Just six years ago, the firm was three separate entities, made up of Chicago's Piper Rudnick, San Diego's Gray Cary Ware & Friedenrich and London's DLA. The firm's geographical spread has enabled it to retain major corporate clients, including Pfizer Inc., Barclays PLC and General Electric Co.

Mainstream firms that want to be full-service but aren't willing to shoulder the risks of a strong overseas presence operate at a disadvantage, said Frank Burch, global board chairman of DLA Piper. "It's a harder game to play," he said.

Typically a firm opens a foreign office to retain an existing client. But moving into international cities can create a recruiting edge, and with new attorneys comes new business. DLA Piper has been able to lure away rainmakers from Skadden, Arps, Slate, Meagher & Flom; Paul Hastings Janofsky & Walker and Nixon Peabody as it has broadened its international platform. K&L Gates recently picked off two finance and corporate partners with international practices from Curtis, Mallet-Prevost.

At the same time, staffing can be one of the biggest challenges of international expansion. The pool of lawyers who know the law within their own foreign jurisdictions and who also can integrate smoothly into U.S. firms is small. The shortage is especially acute in non-European cities, where cultural differences make it more difficult to effectively transplant attorneys from the U.S.

"Integration is the single most important thing you can do to have an office succeed or fail," said Mark Thompson, a partner in the London office of King & Spalding. "The question is how you compete against the larger, stronger incumbents in those markets." he said. "I think it's really hard."

With heavyweight clients needing work in Moscow, King & Spalding had long eyed the Russian capital, but it didn't want to go forward with plans until it had the right mix of lateral partners, said Thompson, who is helping launch the Moscow location. After recruiting four attorneys from Hogan Lovells and one from White & Case, the Moscow office opened in May. Part of the issue was making sure it was positioned to service existing clients, as well as cross-sell to others. "You don't want partners coming back a year later and saying 'What are those people doing over there?'" Thompson said.

Similarly, firm leaders at McDermott Will & Emery had been thinking about opening a Paris office for roughly 10 years, said Peter Sacripanti, co-chairman of the 950-attorney law firm. But it wasn't until McDermott hired former Herbert Smith partner Jacques Buhart to lead the office that the firm felt confident it could take care of its clients."Everything we do is with conservatism," said Sacripanti. The eight-attorney office, which opened in May, will focus on financial services, pharmaceutical, energy and technology work. He expects the office eventually to expand to 35 attorneys.

Over the last few decades, American firms' interest in foreign expansion has moved in spurts. In the 1980s and '90s, the London, Paris and Germany cities were the places to expand. Covington & Burling; McDermott, Will & Emery; Bryan Cave; Winston & Strawn and many others sought corporate, transactional and finance work there.

Hong Kong was also a popular destination, and continued to be during the early 2000s. Troutman Sanders planted a flag in Hong Kong in 1999. Others followed, including Sidley Austin, now-dissolved Heller Ehrman; Baker Botts; Vinson & Elkins; and Bingham McCutchen. In fact, in 2005 the number of attorneys in foreign offices at top U.S. law firms shot up 23 percent, according to The National Law Journal.

This time around, firms are planting flags further to the east. Baker & McKenzie recently opened in Qatar; Vinson & Elkins in Saudi Arabia; and DLA Piper in Mongolia. And they say they are also making more of a business case to the partnership before moving ahead.

Of course, expansion isn't the road to riches for every firm. Two firms with some of highest profits per partner have very small foreign practices. At Paul, Weiss, Rifkind, Garrison & Wharton, known for its litigation practice, only about 50 of its 620 attorneys work outside the United States. The firm's largest foreign offices are in London and Hong Kong, with only about 15 attorneys each. Paul Weiss' profits per partner were up 15% in 2010 to $3.1 million, according to The American Lawyer, making it No. 7 on the AmLaw 100.

And the richest of them all, Wachtell, Lipton Rosen & Katz, with profits per partner of all law firms at $4.3 million, according to The American Lawyer, has its sole office is in New York.

(Reporting by Leigh Jones)