HARARE, Zimbabwe — The rapid fall of Zimbabwe’s president, whose legendary guile and ruthlessness helped him outmaneuver countless adversaries over nearly four decades, probably has surprised no one more than Robert Mugabe himself.

For years, he was so confident of his safety — and his potency — that he took monthlong vacations away from Zimbabwe after Christmas, never facing any threat during his long, predictable absences. Even at 93, his tight grip on the country’s ruling party and his control over the military made his power seem impervious to question.

But in just a matter of days, Mr. Mugabe, who ruled his nation since independence in 1980, was largely stripped of his authority, even as he still clung to the presidency.

In a much-anticipated speech on Sunday night, Mr. Mugabe, instead of announcing his resignation as most of the country had expected, stunned Zimbabwe by refusing to say he was stepping down. While he conceded that his country was “going through a difficult patch,” he gave no sign that he recognized, or accepted, how severely the ground had shifted under him in such a short time.

Earlier in the day, the governing ZANU-PF party, over which he had always exercised total domination, expelled Mr. Mugabe as leader, with cheers and dancing erupting after the vote. He was given a deadline of noon on Monday to resign or face impeachment by Parliament.

Just days earlier, on Wednesday, soldiers put him under house arrest, and his 52-year-old wife, Grace Mugabe, whose ambition to succeed him contributed to his downfall, has not been seen in public since.

But in his speech, Mr. Mugabe even declared that he would preside over his governing party’s congress in a few weeks. After 37 years in control of the nation, he was refusing to let go easily.

A Fateful Firing

The chain of events leading to Mr. Mugabe’s downfall started on Nov. 6, when he fired his vice president, Emmerson Mnangagwa, a close ally of the military, and then tried to arrest the nation’s top military commander a few days later. Mr. Mugabe had finally come down against the military and its political allies in a long-running feud inside the governing party.

“He crossed the red line, and we couldn’t allow that to continue,” said Douglas Mahiya, a leader of the war veterans’ association, a group that has acted as the military’s proxy in the country’s political battles while allowing uniformed generals to remain publicly neutral.

A few hours after he was fired, Mr. Mnangagwa, fearing arrest, fled with a son into neighboring Mozambique, where he has strong military ties. He eventually made his way to South Africa, allies said.

July Moyo, a close ally of Mr. Mnangagwa, said the vice president had prepared himself for the possibility of being fired. “He accepted that things can turn very bad, so he had conditioned himself,” Mr. Moyo said.

Several hours before the vice president escaped to Mozambique, Christopher Mutsvangwa, the head of the war veterans’ association and one of Mr. Mnangagwa’s closest allies, had boarded a plane to South Africa.

Over the following days, Mr. Mutsvangwa met with South Africa intelligence officers, he said, warning them of a possible military intervention in Zimbabwe. He said he had tried to persuade South African officials not to describe any intervention as a “coup” — an important concession to get from South Africa, the regional power.

Though this account could not be verified with South African officials on Sunday, the South African government did not mention the word “coup” in its official statement after the military intervention occurred on Wednesday.

“I knew that the way they were driving, the military, inevitably, there would be one at one stage or another” Mr. Mutsvangwa said, referring to a military intervention.

While Mr. Mutsvangwa worked on South African officials, Zimbabwe’s longtime top military commander, Gen. Constantino Chiwenga, was in China on an official trip. He was tipped off while abroad that Mr. Mugabe had ordered him arrested upon his return home, according to several people close to the military. The police were going to grab the general as soon as his plane touched down, on Nov. 12.

But as General Chiwenga prepared to land, soldiers loyal to him infiltrated the airport. His troops — wearing the uniforms of baggage handlers — surprised and quickly overwhelmed the police. Before departing, the general is said to have told the police officers that he would “deal” with their commander, a Mugabe loyalist.

Within just a couple of days, tanks had rumbled into the capital and soldiers had effectively deposed Mr. Mugabe.

Fierce Infighting

The president’s decision to fire his vice president and arrest the general was the culmination of a long — and increasingly vicious and personal — battle inside ZANU-PF, the party that has controlled Zimbabwe since independence in 1980. The so-called Lacoste faction was led by Mr. Mnangagwa, whose nickname is the Crocodile, and was backed by the military and war veterans.

The rival faction was led by the president’s wife and supported by the police, whose loyalty Mr. Mugabe had ensured by, among other moves, naming a nephew to a top command. This faction included mostly younger politicians with no experience in the war of liberation and was christened Generation 40, or G-40, by Jonathan Moyo, a fearless, extremely ambitious politician widely regarded as the mastermind behind this group.

As Lacoste and G-40 fought each other to eventually succeed Mr. Mugabe, the president did not give either side his declaration of support. To both factions, the biggest factor was Mr. Mugabe’s age and increasingly visible frailty. It was only a matter of time before “nature will take its course” and “the old man goes,” as the political class said.

Time was on Lacoste’s side. Once nature did take its course, power would naturally slip to Mr. Mnangagwa and his military backers, they believed.

Mr. Mnangagwa remained largely quiet, refraining from responding to attacks, and treated Mr. Mugabe with extreme deference. Whenever Mr. Mugabe flew home from a trip, state media invariably showed Mr. Mnangagwa greeting the president on the tarmac, displaying an almost obsequious smile and body language.

To the younger members in G-40, time was against them. Their biggest asset, Mrs. Mugabe, would lose all value once her husband died. So they were in a rush to get a transfer of power while Mr. Mugabe was still alive.

Just a few months ago, Mr. Moyo confided in a friend that he was “less than confident” about G-40’s standing with the president. Despite his efforts to win over the president through Mrs. Mugabe, Mr. Moyo still remained unsure about the “old man’s standing vis-à-vis Mnangagwa and Chiwenga,” said the friend, who spoke on the condition of anonymity because the conservation had been private.

“He felt he had to disqualify Mnangagwa very soon because the old man was still tentative,” the friend said.

The First Lady and the Fall

Mr. Mugabe’s downfall was rooted in his wife’s decision to become a political force in mid-2014, most politicians and experts believe.

“Mrs. Mugabe’s entry into politics caused elite rupture in Zimbabwe,” said Tendai Biti, a lawyer, opposition politician and former finance minister in a coalition government a few years ago. “This coup was the result of a disagreement between people eating at the same table, whereas most coups in Africa are done by people eating under the table and receiving crumbs.”

Why Mrs. Mugabe, now 52, suddenly dove into politics is not exactly clear. Married for decades to Mr. Mugabe, she had been known as “Gucci Grace,” someone interested in shopping and leading a lavish lifestyle. She was a typist in the presidential pool when she and Mr. Mugabe began an affair while the president’s first wife, Sally, was dying of cancer. Unlike the much-beloved first wife, the second Mrs. Mugabe was widely disliked among Zimbabweans.

Some politicians and experts point to the hand of Mr. Moyo, the originator of the G-40 name, for Mrs. Mugabe’s political intentions.

In ZANU-PF’s ever-shifting alliances, Mr. Moyo had a checkered past. In 2004, he was expelled from the party after planning a power play with — critically — none other than Mr. Mnangagwa himself, who managed to escape politically unscathed. Feeling betrayed by Mr. Mnangagwa, Mr. Moyo vowed never to work with him again, setting off a decade-long feud that fed into the recent military takeover.

Mr. Moyo, reportedly admired by Mr. Mugabe for his intelligence, was rehabilitated, rejoined the party and was given ministerial positions in the cabinet.

But in June 2014, Mr. Moyo was again on the outs. At a funeral for a party stalwart at National Heroes Acre, a burial ground and national monument in Harare, the capital, Mr. Mugabe criticized Mr. Moyo for causing dissension in the party. The president referred to him as a “weevil” — an insect that eats corn, Zimbabwe’s staple food, from the inside.

“Even in Zanu-PF, we have the weevils,” the president said. “But should we keep them? No.”

To secure his survival, Mr. Moyo urged Mrs. Mugabe to enter politics, according to politicians, friends and analysts.

“He preyed on her fears,” said Dewa Mavhinga, a Zimbabwe researcher for Human Rights Watch, referring to Mr. Moyo. “You’re a young wife with an old husband in his sunset moments. You have to guarantee your future. You need people who are loyal to you. And who better to protect your interests than yourself.”

Very rapidly, Mrs. Mugabe and her allies orchestrated the removal of rivals, including Joice Mujuru, a vice president, as well as Mr. Mutsvangwa, who had been Mr. Mugabe’s minister of war veterans affairs.

But even as the president’s medical trips to Singapore were getting increasingly frequent, he was not making a final decision on his succession.

Time was running out.

And so, Mr. Moyo, shortly after expressing his growing frustrations to his friend, appeared to go for broke. In July, in a meeting of party leaders, he launched a direct attack on Mr. Mnangagwa, presenting a 72-minute video said to show his rival’s duplicity and desire to topple the president.

At the same time, Mrs. Mugabe intensified her faction’s attacks, describing Mr. Mnangagwa as a “coward” and “coup plotter.”

At a rally in the city of Bulawayo early this month, some youths, presumably from the rival Lacoste faction, began heckling Mrs. Mugabe, calling her a “thief.”

“If you were paid to boo me, go ahead,” she said. “I am the first lady, and I will stand for the truth. Bring the soldiers and let them shoot me.”

The heckling visibly angered Mr. Mugabe, who immediately accused Mr. Mnangagwa of being behind it.

“Did I err in appointing Mr. Mnangagwa as my deputy?” the president said. “If I erred, I will drop him even tomorrow.”

Two days later, he fired Mr. Mnangagwa, opening the path for Mrs. Mugabe to become vice president and, once nature took its course, her husband’s successor.

Mrs. Mugabe and her allies had finally won. But the victory would soon prove Pyrrhic.

As the Lacoste faction solidified the takedown of Mr. Mugabe, party officials on Sunday removed Mrs. Mugabe as head of the ZANU-PF Women’s League and barred her from the party for life. Mr. Moyo, too, was barred forever. Mr. Mugabe’s second vice president, Phelekezela Mphoko, who had served for three years, was fired.

The ending was much sweeter for Mr. Mnangagwa: On Sunday, the party named him as its new leader.