NEW YORK-Maurice R. Greenberg may not be indispensable to American International Group Inc., but for decades he seemed that way.

As he built AIG from a company with $13.4 million in net income in 1969 to one with $11.1 billion last year, Mr. Greenberg, who resigned under pressure last week as AIG's chief executive officer, stood out in an industry not noted for large personalities.

Famously autocratic, demanding and short-tempered, Mr. Greenberg, widely known as "Hank," fostered a corporate culture that encouraged decisiveness, required accountability and rewarded success. He was also, less famously, cordial and generous with executives who thrived in that culture, former AIG officials say.

"He was probably the most demanding boss I ever worked for, but probably the fairest guy," said Joseph Wiedemann, a former senior executive of several AIG units. "He rewarded you for the results you had, and he rewarded you very well."

Known for his minute grasp of AIG's byzantine operations-leaving some startled AIG executives with the impression that he knew more about their businesses than they did-Mr. Greenberg has also been a deft strategic thinker, industry observers say. He led AIG's transformation from a collection of property/casualty and life insurers into a financial services conglomerate, adding retirement products, asset management and consumer and aircraft financing operations in a series of multibillion-dollar acquisitions.

He also pushed an international expansion that saw AIG-which traces its beginnings to Shanghai in 1919-become the first foreign insurer to open offices in Communist China and one of the first to enter Eastern Europe and countries of the former Soviet Union.

Mr. Greenberg's combativeness, though-a posture that served AIG's interests well during its expansion and in earlier confrontations with regulators-may have sped his departure, as AIG faced mounting pressure from New York Attorney General Eliot Spitzer and the U.S. Securities and Exchange Commission over issues ranging from AIG's role in alleged bid rigging by Marsh Inc. to the insurer's own reinsurance transactions.

"It's wrong to talk in the past tense" about Mr. Greenberg, who for now remains AIG's nonexecutive chairman, observed Myron Picoult, an independent insurance consultant in New York.

Mr. Picoult added, though, that "there's only been one Hank Greenberg. I doubt if you'll ever see the likes of him again."

While driven to see AIG rise above its competitors, Mr. Greenberg has seemed reluctant in various interviews to talk about the sources of that drive.

A few biographical details are widely reported: He grew up during the Depression on a farm in upstate New York, leaving at age 17 to join the U.S. Army and landing in Normandy during the Allied invasion of Europe in 1944. After the war, he returned to the United States to complete college and graduate from New York Law School in 1950. Still a member of the Army Reserve, he served in the Korean War, rising to the rank of captain and winning a Bronze Star.

Beginnings at AIG

After a stint at Continental Casualty Co., Mr. Greenberg was recruited in 1960 by Cornelius Vander Starr, who had launched the Shanghai insurance agency that became AIG and who was looking for someone to build an international accident and health operation.

Two years later, Mr. Greenberg was named president of American Home Assurance Co. and quickly turned the insurer upside down, scrapping the expensive agency force that had limited its underwriting control and turning it into a broker market for large commercial risks. By 1967, Mr. Greenberg had been named AIG's president and CEO, and Mr. Starr declared him his successor shortly before he died and shortly before AIG became a publicly traded company in 1969.

Over the next 35 years, Mr. Greenberg oversaw AIG's explosive growth. In 1969, the company wrote net property/casualty premiums of $245.5 million and life premiums of $38 million; last year, net property/casualty premiums totaled $41.9 billion, life premiums reached $28.4 billion and AIG's overall revenues amounted to $98.61 billion.

The 1980s saw AIG's rapid expansion into financial services, with the acquisition or startup of asset management, currency and commodity trading and finance units. In 1994, AIG acquired 20th Century Industries, moving into personal lines; four years later, in an $18 billion deal, AIG acquired SunAmerica Inc., putting it in the retirement savings business. Its biggest acquisition, in 2001, was the $23.2 billion takeover of life insurance and pension company American General Corp.

Another milestone came in 1992, when AIG became the first foreign insurer to be granted a license to operate independently in Shanghai. AIG had closed its offices in China in 1950 after the Communist takeover. Its return was the culmination of a long campaign by Mr. Greenberg that included a 1980 joint venture with the state-run People's Insurance Co. of China and the 1990 formation of an international business advisory council in Shanghai to consult with the government on financial services development. Shanghai's mayor at the time-Zhu Rongji, later China's premier-appointed Mr. Greenberg chairman of the council.

Mr. Greenberg managed AIG's expansion and its ongoing business in a style that industry observers say is not likely to be duplicated.

"It's amazing-not only what he created but his absolutely phenomenal grasp of what was going on in his company," Mr. Picoult said.

Mr. Greenberg kept a sharp eye on his managers' performance and was not shy about dressing them down in blunt language for mistakes. At the same time, though, AIG's various units operated independently, and Mr. Greenberg expected his executives to run their own businesses.

"He spawned a very tough culture, but it's a culture of decision makers," Mr. Picoult said. "You're paid to make decisions at AIG."

William L. Munson, who held several senior management positions in 13 years at AIG, contrasted the company's trajectory with that of Home Insurance Co. Home-now in liquidation-was the first U.S. insurer to open an office in Germany in 1870, was operating in China in the early 1900s and was the largest U.S. insurer in the 1940s, he noted.

"Look at Home and look at AIG today," Mr. Munson said. "What's the difference? Management."

"He was a tough taskmaster," he said of Mr. Greenberg. "Very quick, very tough, very decisive...You didn't see him make a bad decision when he had the right information."

A different side

He was also generous to loyal AIG employees in ways that went beyond compensation. Mr. Wiedemann recalled that Mr. Greenberg was the first person he heard from after Mr. Wiedemann's wife suffered a stroke while the two were traveling. Mr. Greenberg offered a private jet to return them to New York and made an appointment for Ms. Wiedemann with his own cardiologist, Mr. Wiedemann said.

"Most people didn't see that side," he said.

Indeed, the tough attitude was more in evidence and has long been a subject of mordant humor among AIG veterans.

Brian Duperreault, chairman of Bermuda-based ACE Ltd., spent 21 years in various senior positions at AIG before joining ACE. In January, when accepting an industry award in New York, he singled out Mr. Greenberg.

"If there's anybody I need to thank it would be Hank...I learned a lot from him, especially how stupid I was," Mr. Duperreault joked.

Mr. Greenberg's combative nature and iron grip on the helm at AIG contributed to the steady departure of senior executives over the years; AIG became known for producing executives who, like Mr. Duperreault, left to head other companies.

Among that group were two of Mr. Greenberg's own sons, Jeffrey and Evan. Both rose to upper management positions after years at AIG and were considered at varying times to be Mr. Greenberg's likely successor. Jeffrey quit in 1995 and later became CEO of Marsh & McLennan Cos. Inc., a position he was forced to resign last year by Mr. Spitzer's fraud and bid-rigging charges against the broker. Evan followed Jeffrey out the door in 2000, later becoming CEO of ACE.

Demanding with employees and hard on competitors, Mr. Greenberg was also, until recently, able to face down regulators that he felt were interfering unjustifiably in AIG's business.

One of his most widely publicized run-ins grew out of investigations in the 1990s of Coral Reinsurance Co., a Barbados reinsurer that AIG helped form specifically to reinsure its units but that it claimed not to control. Regulators in Pennsylvania, New York and Delaware labeled the Coral reinsurance "window dressing" designed to boost AIG's surplus, but the insurer emerged largely victorious after months of battling regulators' objections: it suffered no penalty other than being required to unwind its business with the reinsurer.

AIG's success in wearing down opponents may have led Mr. Greenberg to underestimate the seriousness of the series of inquiries that began more than a year ago and that has included settlements with the SEC over allegedly fraudulent financial insurance transactions and Mr. Spitzer's ongoing investigation. The rising pressure reportedly led AIG's board to demand Mr. Greenberg's resignation as CEO last week.

Regardless of the outcome, the culture Mr. Greenberg created in four decades at AIG is unlikely to change quickly under new CEO Martin J. Sullivan, observers say.

"There's nothing that says that culture cannot be sustained," Mr. Picoult said, adding that Mr. Sullivan, while more genial than Mr. Greenberg, may prove "just as tough or tougher than Hank."

"The culture that he laid out for the company will take a long time to wear off," Mr. Wiedemann agreed.