Defense Minister Kim Yong-hyun walks toward a meeting room at the Government Complex in Seoul, Tuesday. Kim faces impeachment as well as criminal charges of treason, an offense punishable by life in prison or death, following President Yoon Suk Yeol's short-lived declaration of martial law. Yonhap

President Yoon Suk Yeol's declaration of martial law immediately incited political chaos.

Soon after the late-night announcement on Tuesday, Army Chief of Staff Gen. Park An-su, who was named his martial law commander, issued measures under the decree, including a ban on "all political activities" and restrictions on press freedom. Within about two hours, more than 200 armed troops entered the buildings of the National Assembly.

Almost everyone was shocked, including Prime Minister Han Duck-soo, one of many Cabinet members who had apparently been unaware of the plan until the last minute — but not Kim Yong-hyun, Yoon's defense minister who had planned the bid.

Speaking to reporters, Wednesday, a ministry official confirmed that it was Kim who asked Yoon to issue a martial law order. This comes hours after lawmakers unanimously voted to nullify it.

Now, Kim faces impeachment as well as criminal charges of treason, an offense punishable by life in prison or death.

The main opposition Democratic Party of Korea (DPK) said it is seeking to remove Kim and his boss, Yoon, as well as Interior Minister Lee Sang-min from office over the plot for an "unconstitutional and unlawful" martial law declaration.

It is unclear whether Lee was also informed of it in advance. The party will also file a criminal complaint against Kim, accusing him of pushing for martial law without sufficient legal basis.

"The DPK will punish the unconstitutional and illegal emergency martial law of the Yoon administration," Jo Seoung-lae, the party's chief spokesperson, said in a statement. "We urge law enforcement agencies to immediately launch an investigation into the treason case that the entire nation is now aware of and bring the perpetrators to justice."

With the ruling People Power Party largely sharing the DPK's views on what should be done to punish Kim for his role, the impeachment and investigation targeting him are expected to proceed fast.

The Constitution stipulates that a president may declare martial law in response to "war or war-like emergency situations" when the government's administrative and judicial branches cannot function amid chaos.

Many legal experts, including Cha Jin-ah, a law professor at Korea University, and Han In-sup of Seoul National University School of Law, said that Yoon, Kim and others involved in making the decision abused their authority beyond the limit of what the Constitution allows.

In a statement released on Wednesday evening, Kim apologized for his role in causing social and political tumult and offered to resign as defense minister.

"First, as defense minister, I feel deeply responsible and sorry for causing concern and confusion in regard to the martial law," he said. "All members of the armed forces involved in the implementation of the emergency martial law only did their duty at my direction. I take full responsibility for it."

Kim, 65, an alum of Choongam High School, from which Yoon also graduated, was appointed as the administration's third defense minister only three months ago.

During his confirmation hearing at the Assembly on Sept. 2, Kim dismissed any possibility of a martial law decree in response to opposition lawmakers pressing him on the issue.

"I think talk about martial law is out of date," he said. "If it is declared, who would accept it? Do you think the military would even follow the order?"

Over the last three months, he allegedly plotted military rule under a martial law decree and asked the president to implement it. All this suggests that he began to map it out soon after assuming the post.

Rep. Kim Min-seok, one of the DPK lawmakers criticized for spreading conspiracy theories about the possibility of martial law, expressed concerns that this may not be the end, warning that Yoon could make another martial law attempt amid mounting political and legal pressure.