Late in the 16th century, after a century of continual civil war, Japan was adrift and in despair. Out of nowhere came a lowly samurai named Oda Nobunaga, who won a series of brilliant victories, overcame the corrupt aristocracy and dominated Japan.

He was an aesthete, art patron and merciless killer. His most notorious act of brutality was to burn down 3,000 Buddhist temples outside Kyoto and butcher their inhabitants. And 400 years later, Oda Nobunaga is a source of inspiration, if not a role model.

"I am learning greatly about the harsh life of a samurai warlord," Mr Koizumi told The Times in his first interview since winning an election that has turned Japan’s political order on its head. "Every day they faced death. There are a lot of lessons to be learnt." Like his samurai exemplar, Mr Koizumi has risen from relative obscurity to set in motion a transformation of Japanese politics.

After taking office in 2001 during Japan’s worst postwar recession, he has overhauled the country’s debt-stricken banks, presided over a modest economic recovery and sent troops to Iraq, overcoming a half-century taboo on the dispatch of Japanese military force overseas. In the past few weeks he has carried out a political purge as devastating, in its way, as Oda Nobunaga’s liquidation of the monks of Kyoto.

It began last month with the defeat in the Upper House of Japan’s Diet of legislation that would have brought about Mr Koizumi’s lifelong political goal — the privatisation of Japan’s post office services. To the dismay of his friends, as well as enemies, the Prime Minister forced a snap election. His Liberal Democratic Party split as opponents of the postal Bills deserted their leader to run as rebel independents.

Mr Koizumi fought the election on a simple set of either/or choices. Postal privatisation or not? Continuing structural reform or a return to the old ways? With me — or with the rebels? "This election was about asking the Japanese people to turn around the Diet’s decision," he says.

The result was a massacre. The LDP leapt from 212 MPs to 296; together with their small coalition partner, they control 327 out of 480 Lower House seats. Half of the rebels were wiped out, and 83 new MPs, most of them fervent Koizumi supporters, were elected.

Yesterday, in an exclusive interview in his official residence, he talked about his bitter victory, about the exhausting war against his party enemies, and about the tension between Japan and its neighbours caused by his visits to the nationalist Yasukuni Shrine.

Who could name any of the seven prime ministers who came and went in the ten years before Mr Koizumi? Traditionally it was a job passed from one grey man to the next, and between the competing factions that made up the LDP. Mr Koizumi has put a personal stamp on the office.

It is on his orders that his aides have taken what for Japanese bureaucrats is a radical step: casting off their jackets and ties in favour of open-necked shirts, intended to reduce the need for air conditioning.

Perhaps because of the presence of The Times photographer, the Prime Minister wears a tie today. He talks for 40 minutes in his staccato style, punctuated by jabbing, emphatic hand gestures.

Mr Koizumi’s obsession with postal reform goes back decades. As a young politician he wrote not one, but two, books on the subject (Reasons to Reassemble the Ministry of Posts and Telecommunications was followed by Reasons to Privatise Posts and Telecommunications). "I have been saying all along that my top priority has been the privatisation of Japan Post," he says. "I have called it the bastion of reform. I said that if this can be realised it will be a political miracle. Now it looks likely that the miracle will happen."

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