LONDON, NOV. 22 -- For more than a decade, Margaret Thatcher defined what leadership in the modern age was all about. She breathed new life into a Britain that seemed in the grip of terminal decline, preaching the gospel of free-market economics, breaking the power of the trade unions and epitomizing a philosophy that came to bear her name -- Thatcherism.

She strode the world stage, setting an example of resolution, winning respect and admiration and exercising influence out of all proportion to the size and power of the small island nation she led. All of which made the timing of her resignation cruelly ironic. For Thatcher's political career was being snuffed out this week just as she was celebrating the triumphant vindication of her most cherished values at the European security summit in Paris.

Thatcher lacked the superpower mantle of Mikhail Gorbachev, the economic clout of Helmut Kohl or the easy charm of Ronald Reagan. But she ruthlessly seized history, capitalized on moments like the 1982 Falklands War that tested her character and will and became ultimately the West's most forceful spokesman for the virtues of capitalism, individual rights and strong defense that were celebrated this week in Paris.

She was ardent, tenacious and combative, a visionary who scorned consensus politics and conventional wisdom, a restless revolutionary who even when isolated believed in her own innate rightness. Those were the qualities that defined her greatness -- and finally, those were the qualities that destroyed her.

"The Greeks understood it all," said former foreign secretary David Owen, a longtime opponent yet an admirer. "Great men and women are not brought down by lesser mortals, they are brought down by themselves. Margaret Thatcher was never going to be slain by a Geoffrey Howe or a Michael Heseltine, but she could always kill herself."

Thatcher infuriated many Britons with her hectoring, schoolmarm style, her constant use of the regal "we" when referring to herself, her unmodulated intensity and her dogged lack of humor. She never won more than 43 percent of the vote in any of her three general election victories -- benefiting from politically suicidal divisions among her opponents -- and poll after poll indicated that most Britons still supported the welfare state and semi-socialistic system she despised and relentlessly sought to dismantle.

Yet many admired her courage, thrilled to her triumphs and responded to her leadership with the same tolerance they had shown 50 years earlier for another complex, difficult and demanding prime minister -- Winston Churchill.

"Both Thatcher and Churchill had a very clear sense that consensus politics can mean taking the line of least resistance and that compromise can inevitably lead to appeasement and weakness," said historian Martin Gilbert, author of an eight-volume biography of Churchill.

"Both of them also understood that the essence of leadership was to choose a course of action you knew to be the right one and stay with it, and not try to achieve compromise in order to have an easy ride whether in cabinet or Parliament or with the editorial writers."

Thatcher herself put it in even more combative terms: "I must say the adrenalin flows when they really come out fighting at me and I fight back, and I stand there, and I know 'Now come on, Maggie, you are wholly on your own; no one can help you.' And I love it."

Gilbert believes Thatcher also mirrored Churchill in her innate sense of how best to deal with the Soviet Union. Just as Churchill sought, after the death of Joseph Stalin in 1953, to use the enormous military advantage of the West to compel Moscow to strike a disarmament deal, so she sought to use the deployment of U.S. Pershing missiles in Europe and president Reagan's Strategic Defense Initiative to push Stalin's heirs to the table.

"She understood that you don't say, 'Oh dear, we're so weak, swallow us last.' You say, 'Look, we are very strong, so let's come to terms,' " said Gilbert.

Thatcher's strength was most recently on display in the Persian Gulf crisis. She was in Aspen, Colo., visiting with President Bush when the crisis began, and she encouraged Bush to take a hard line. Britain was the first Western state to back the United States by committing aircraft and troops to the international force arrayed against Iraqi aggression. And while Bush at times has sounded muddled and contradictory when talking about war aims, Thatcher has been consistently direct, tough and uncompromising.

One example came two weeks ago when Secretary of State James A. Baker III passed through London on a European tour designed to win support for a new U.N. Security Council resolution authorizing the use of force to liberate Kuwait. Thatcher made clear she would support the move, but also that she believed it totally unnecessary. Clarity of purpose and a demonstration of will were needed far more than further embroidering of the international consensus, she argued.

Thatcher benefited not only from the fratricidal divisions among her opponents, but also from the unique powers the British political system bestows on a modern prime minister. As Financial Times columnist Samuel Brittan pointed out, she was not only leader of her government and leader of her party in Parliament, she was also in sole control of vast lists of patronage appointments. Unlike Bush, she was not hampered by a legislature controlled by her opponents; unlike Helmut Kohl or Yitzhak Shamir, she did not need to concern herself with the agendas of junior coalition partners.

"We are the only strong system of government in all of Western Europe," one of her senior aides insisted, "and the only one where tough decisions can be made without constantly having to worry about being undermined."

The British system provides for a government cabinet in which the prime minister is, at least in theory, first among equals. But over Thatcher's 11 years in power, critics complained that cabinet government became something of a private joke. Many of the strongest personalities left, driven out by her insistence on personally controlling policy or, as in the case of former chancellor of the exchequer Nigel Lawson, undermined by her own private advisers. The two men who finally destroyed her -- former defense secretary Michael Heseltine and former deputy prime minister Geoffrey Howe -- both walked out of her cabinet for similar reasons.

In the end, she was left with a cabinet that was often reluctant to contradict or challenge her, a group of younger men who were in many ways articulate and attractive but far less experienced than she had become. And their failure to stand up to her on what seemed a relatively minor domestic issue ultimately led to her downfall.

Britain's sharp economic downturn plagued Thatcher in her last year in office and might have cost her the next election. Divisions over the country's future role in Europe haunted her government and her party. But it was her unswerving advocacy of a new system of local taxation commonly called the poll tax that many analysts agree was most responsible for her demise.

Like so many of the causes she championed, the poll tax was given birth by a small group of right-wing idealists who saw it as the next logical step of Thatcherism. They wanted to correct an alleged defect of democracy in the system of financing local government through property taxes -- those who did not own property did not pay, yet by their votes they could influence local government spending.

By introducing a flat-rate tax on all adults, the Thatcherites believed they could clip the wings of high-spending, opposition-dominated local councils. But the measure was deeply unpopular with middle-class homeowners who had been paying modest property taxes but who suddenly found themselves faced with poll-tax bills for double or triple their previous assessments.

Thatcher had been warned by some Tory lawmakers that the measure would backfire. She rammed it through because she believed it was right. And when government estimates of revenues from the new tax proved woefully low, she did not flinch. She did exactly what she had done with the Falklands and in every other crisis -- she fought on.

"The poll tax was the big strategic error," said pollster Robert Worcester, chairman of Marketing & Opinion Research International. "Every politician knows you can't alienate your base, but she did. She wouldn't listen to her party or her colleagues or the polls."

It reached a point last spring at which Thatcher's approval rating in Worcester's polls dropped to 20 percent, the lowest in British history. Thatcher never faltered. Aides say she returned from Paris Wednesday convinced she could win the second round of her bruising leadership battle with Heseltine. But a parade of cabinet ministers told her otherwise. This time, she heeded them.

After it was all over and she had tendered her resignation to the queen, Thatcher disdainfully lashed at her opponents one more time in the House of Commons. "For them it's all compromise, sweep it under the carpet, leave it for another day; it might sort itself out," she said. By contrast, she added: "It is because we on this side have never flinched from difficult decisions that this house and this country can have confidence in this government today."