Explained | South Korea's leadership crisis

President Yoon Suk Yeol's short-lived declaration of martial law last month has created South Korea's biggest constitutional crisis since the country democratized in the late 1980s.

On Wednesday, Yoon became the first sitting South Korean leader to be detained for questioning by criminal investigators, who want to question him over his decision on December 3 to put the country under military rule for the first time in 45 years.

His martial law decree lasted only six hours. But it threw South Korea's democracy into chaos and drew public outrage, recalling the country's painful history of military dictatorship decades ago. He was impeached by parliament on December 14, but the uncertainty over the country's political future has only deepened since.

Here is how the crisis has unfolded:

What are the criminal accusations against the president?

Yoon, a deeply unpopular leader, has been banned from leaving the country as the police and prosecutors investigate ​whether he and his supporters in the government and military committed insurrection when they sent armed troops into the National Assembly.

Col. Kim Hyun-tae, who led a special forces unit that was sent into the assembly, has said that he received orders to forcibly remove lawmakers ​to prevent 150 of them -- the number required to repeal martial law -- from gathering.

South Korea's criminal ​law defines insurrection as any attempt to "overthrow government organs established by the constitution or to render the exercise of their functions impossible by force."

Now that they have detained Yoon, investigators can question him for 48 hours. They could then apply for a separate court warrant if they want to formally arrest him.

If ​Yoon were to be convicted of insurrection and the court ​ruled that he was its ringleader, he​ could face the death penalty or life imprisonment. Prosecutors have arrested his former defense minister​ and two former police chiefs on charges of helping to carry out an insurrection.

Why was Yoon impeached?

In the impeachment bill, opposition lawmakers argued that Yoon had perpetrated an insurrection when he made the martial law declaration and sent troops into the National Assembly. They said that was an attempt to stop lawmakers from voting down the decree, as was their right under the constitution.

An initial impeachment vote on Dec. 7 failed, after lawmakers from Yoon's People Power Party boycotted it, saying that he should be given a chance to resign.

A week later, 12 lawmakers from Yoon's party joined the opposition to impeach him.

Now his fate as president lies in the Constitutional Court, which has begun deliberating whether to remove or reinstate him. The court's deliberations can take up to six months.

Why did Yoon do it?

Yoon has said that he declared ​martial law out of "desperation​" in the face of an opposition that used its parliamentary majority to "paralyze" his government. Yoon ​criticized the opposition for slashing some of his government budgets planned for next year​, as well as its frequent attempts to impeach his political appointees.

But such complaints cannot be grounds for declaring martial law, Kim Young Hoon, head of the Korean Bar Association, told The New York Times. Yoon also did not immediately notify the National Assembly of his declaration of martial law, as required by law, according to the assembly.

"It's clear that President Yoon's declaration of martial law failed to meet the requirements set by the Constitution," Kim said​.

Cho Ji-ho, head of the National Police Agency, ​told the assembly Dec. 9 that when martial law was briefly in place, the military asked the police to help it locate and detain 15 people, including the leaders of the biggest political parties​.

Even during martial law, however, the president has no right to detain lawmakers unless they are caught committing a crime.

Who is in charge now?

Choi Sang-mok, the finance minister, was named the new acting president Dec. 27.

Choi is not an elected official. He is leading South Korea with no real political heft as the country faces challenges such as North Korea's growing nuclear threat and the return of the unpredictable Donald Trump to the White House.

Choi was appointed after Prime Minister Han Duck-soo, who had been serving in the interim role after Yoon's impeachment, was himself impeached by lawmakers in a standoff over the appointment of justices to the court that will decide Yoon's fate.

Yoon, meanwhile, has vowed to fight in court to regain his power.

In a recorded speech released shortly after his impeachment, Yoon listed what he considered his accomplishments as president, including his efforts to strengthen military ties with the United States and Japan. Now, his efforts have been paused, he said.

"But I will never give up," he said.

The court will decide within 180 days whether Yoon is guilty of the crimes the National Assembly accused him of and, if so, whether they are serious enough to merit removal. If the court does formally remove him, South Korea should hold elections for a new leader within two months.