The fall of the House of Thatcher has appeared incomprehensible to many people outside Britain. How could the British behave in this way while their leader was attending such an important international conference in Paris?

This was the question asked all over the world as dissident former Cabinet Minister Michael Heseltine stood against Margaret Thatcher for the leadership of the Conservative Party while she was at the Conference on Security and Cooperation in Europe this week.The answer is simple: Thatcher deliberately chose to have the election for the leadership - the election that led to her downfall - on the day she was in Paris. Under the rules of the Conservative Party, the leader can be challenged every year at around this time. But it was Thatcher who deliberately brought forward the day after being challenged by Heseltine. It was a deliberate election ploy, so that people should associate her with the world stage.

There are many ironies attached to the story of the fall of the House of Thatcher. One is that she herself was the first beneficiary of the rule by which any Conservative member of Parliament, given a proposer and seconder, can challenge the leader once a year.

It is true that Thatcher challenged her predecessor, Edward Heath, when the party was in opposition and after Heath had lost a general election. But it is worth noting that the ruthless Thatcher challenged Heath when he was down, whereas Heseltine was hardly challenging a weak opponent. Heseltine had everything to lose.

The common factor in all this is that the Conservative Party is loyal to a leader right up to the moment when it believes that leader is incapable of winning an election. The late Harold Macmillan was deposed in 1963 for this reason - illness being the pretext. (Macmillan lived on for another quarter of a century, thereby proving something.)

It was because many members of the party disliked the way that Macmillan's successor, Lord Home, had been chosen in an undemocratic manner ("old Etonians going through the customary processes of consultating one another," as one critic at the time put it) that the Conservatives introduced their internal parliamentary selection process.

Those who live by the secret ballot shall die by it.

Thatcher, whose lieutenants fought a vicious and underhanded campaign in 1974-75, fell victim to a brilliantly conducted operation by former Defense Secretary Heseltine, who resigned from her Cabinet in 1986 after a row over a relatively minor issue of industrial policy that he managed to turn into a "European" issue.

Thatcher's anti-European attitudes were the casus belli of Heseltine's challenge this week, former Foreign Secretary Sir Geoffrey Howe having prepared the way with a devastating resignation speech in which he pointed the gun at Thatcher over her anti-Europeanism.

But it is more than that. What people will gradually learn about, as the truth floods out, is how Thatcher, who bestrode the world stage like a female colossus, has actually driven almost everyone in the British government with whom she has had dealings to distraction.

Howe qualifies for the title "most patient and loyal man in Britain" - but even he eventually had had enough.

The fatal flaw in Thatcher's character was the arrogance that made her believe her own myth. She really thought that she had created an economic miracle when she had done nothing of the sort. And she tempted providence by pressing ahead with the infamous poll tax that even her most trusted advisers warned would rebound on her.

Heseltine was able to capitalize on her anti-European attitudes and on the poll tax. Polls indicated that the 15 percent lead enjoyed by Labor was largely an anti-Thatcher phenomenon and that under Heseltine the Conservatives could well defeat Labor - despite the high level of inflation and the recent rise in unemployment.

In the first ballot, Thatcher actually won more votes than Heseltine but did not achieve the margins prescribed by the rules. At first she said she would carry on and was only persuaded to stand down by forceful representations from a majority of her Cabinet led by Foreign Secretary Douglas Hurd and Chancellor of the Exchequer John Major.

Since Hurd and Major had proposed and seconded her in the first ballot, they could hardly stand against her. Now, with Thatcher out of the race, they have declared their interest. Next Tuesday, Conservatives will have to choose from Heseltine, Hurd and Major in the second ballot; an indecisive result could lead to a third ballot.

Heseltine has so far conducted an impressive, presidential-style campaign and ought to emerge the winner. But, whichever of the three does win, one of the first things he should attend to is the absurd British system whereby there is no theoretical limit on the length of time a prime minister can serve - unlike the procedure under such real presidential systems as those in the United States and France. The final irony is that if Thatcher had been forced by a constitution to depart after two terms, she would have gone out in style rather than ignominy.