Seoul, South Korea (CNN) South Koreans woke to a political reality Saturday that seemed improbable if not impossible only months ago.

In downtown Seoul, the giant smiling face of Kim Jong Un stared down at pedestrians from a billboard-sized TV screen.

Usually when the North Korean leader appears on screens in the South Korean capital, it is a sign of danger and high tensions on the Korean Peninsula: nuclear tests, missile launches, or promises of annihilation.

But on Friday, in a moment that few had ever expected to see, Kim was talking of peace and reconciliation.

The image of Kim shaking hands with South Korean President Moon Jae-in , the first meeting between leaders of the two countries in more than ten years, could be seen everywhere in Seoul again Saturday, a reminder that, yes, it really did happen.

The two leaders "solemnly declared before the 80 million Korean people and the whole world that there will be no more war on the Korean Peninsula and thus a new era of peace has begun."

"There is no more war," read the headline of Korea's Hankyoreh newspaper Saturday, underneath a full-page photo of Kim and Moon, hand in hand stepping over the military demarcation line (MDL) which has separated the two countries since the end of fighting in the Korean War in 1953.

A big screen in Seoul shows live footage of Kim and Moon's joint news conference Friday.

The landmark deal signed by the two leaders Friday wasn't the Berlin Wall coming down -- the demilitarized zone (DMZ) between the Koreas remains, and there are many, many issues to solve before anything approaching reunification -- but it had something of that historical weight, and has left even the most cynical of South Koreans feeling genuinely optimistic about their country's future.

Pent up emotion

For generations of men and women South of the border, the war and the accompanying threats of military attack have been an ever-present fact of life.

There are more than 3,000 bomb shelters in Seoul, every man between the ages of 18 to 35 has to serve in the military for two years, and the city plays host to tens of thousands of US troops.

The front page of a newspaper in Seoul on Saturday.

No South Korean will tell you they are unaware of the dangers, but at times the capital can feel wholly removed from its belligerent neighbor, existing in another world, even when there is talk of artillery shells raining down on the city from Pyongyang.

Before Friday, many would have said this was because South Koreans just want to get on with their lives -- they didn't build the world's eleventh largest economy by worrying about war everyday -- but the reaction to Kim and Moon's handshake hinted at a degree of pent up anxiety and denial, which for some came out in floods of emotion.

"In theory, we have an armistice but the atmosphere ... is actually quite frightening when there's nuclear tests and missile launches," Jang Gyu-won, 23, told CNN.

In central Seoul, where crowds had amassed the previous day to cheer the announcement, the mood Saturday morning remained jubilant.

South Koreans watch the summit from a train station in Seoul on Friday.

Lee Bong-joo, 52, whose late father was from North Korea, said she was "so happy such an occasion took place."

"I hope there will be good news soon so I can go to my father's hometown and do a memorial service for him there," she said. "He wanted to meet his separated family (in the North), but couldn't, and passed away while waiting. He really wanted to go back to his hometown."

Friday's agreement also included a commitment to reunite families separated by the war with family reunion programs to resume on later this year.

On the other side of the DMZ, North Koreans, who have been kept largely in the dark throughout the apparent rapprochement, were also waking up to the news of the successful summit.

"The historic Panmunjom Declaration accomplished a comprehensive and groundbreaking development in the North-South relationship in line with the whole nation's demand and desire for peace and the unification of the Korean Peninsula," the state-run Korean Central News Agency said, referring to the agreement by it's official name.

A commuter in Seoul picks up a copy of an extra edition of the Munhwa Ilbo newspaper featuring an image of Moon and Kim on Friday.

What happens next

But promising peace and full denuclearization -- as the Panmunjom Declaration does -- is one thing, achieving it is quite another.

Friday's meeting was all about the Koreas, and the declaration includes language about the importance of the countries deciding their future "on their own accord," but a true solution to the crisis on the peninsula will require buy-in from the US, China and much of the rest of the international community, which has subjected North Korea to stringent economic sanctions as it sought to build its nuclear program.

Kim has already met with Chinese President Xi Jinping, and will sit down for an historic summit with US President Donald Trump later this year, placing the fate of the Koreas largely in the hands of the US leader.

Trump -- who many, including Moon himself, have given credit for the meeting taking place at all -- was triumphant on Twitter, writing "KOREAN WAR TO END!" and praising Xi for his assistance.

Chinese Foreign Ministry spokeswoman Hua Chunying applauded the "historic moment of the North and South Korean leaders crossing the military demarcation line" and quoted an ancient Chinese poem: "Disasters are never powerful enough to separate real brothers, and a smile is all they need to eliminate the hard feelings."

A peace treaty to officially end the Korean War may be the easiest thing to arrange, but even that will require many more parties than just the Koreas. When war broke out on the Peninsula in 1950, the US, under the banner of the United Nations, and China were both major belligerents.

Both are parties to the armistice agreement, signed in 1953, which ended hostilities, along with North Korea but not South Korea, whose strongman leader Syngman Rhee refused to sign, though he could not stop his US sponsors from doing so.

Under the new Panmunjom Declaration, Moon and Kim agreed to work towards "quadrilateral meetings involving the two Koreas and the United States and China with a view to declaring an end to the War and establishing a permanent and solid peace regime."

Travis Jeppesen, author of "See You Again in Pyongyang," said "in the coming months, we can expect more North-South engagement in the realms of sports and culture, and eventually commerce, education, and tourism."

"These steps, combined with diplomatic engagement, will slowly but surely lead to the empowerment of the North Korean people," he added.

Koreans in Japan celebrate the summit in a rally Friday.

Major sticking point

The US and China have both indicated support for formally ending the war, but it remains to be seen if the much larger hurdle of denuclearization will stymie those efforts.

What all parties exactly mean by denuclearization has always been a sticking point in discussions between North and South Korea, and North Korea and the US.

The Koreas have reached agreements in the past. They have hosted successful summits in the past, and raised hands and pledged cooperation and solidarity, but the fundamental disputes have remained unsolved, eventually scuttling any momentum towards true peace.

Speaking to CNN at a secure location in Seoul before the summit, North Korea's former deputy ambassador to London, Thae Yong-ho, warned Pyongyang may not be willing to countenance the type of stringent inspections needed to verify full denuclearization.

"I really doubt whether America can force Kim Jong Un to accept the complete, verifiable and irreversible denuclearization (CVID) principle," he said.

"For North Korea, if they accept the CVID then that's the end of the North Korean system."

He explained that true CVID requires inspectors to be given unfettered access to the country, without restrictions or prior approval. North Korea, with multiple large camps for political prisoners, and top secret military bases, would be unlikely to allow inspectors to roam around unsupervised.

"There are many places North Korea cannot actually open to the world," he said. "How can you be sure that North Korea does not hide its (intercontinental ballistic missiles) in those areas? Real and genuine CVID must mean complete access."

A pro-unification activist in Seoul holds a bottle of ginseng wine from the North Korean city of Kaesong on Friday.

Vicious cycle

Pyongyang will also argue, justifiably many would say, that it should not have to give up its nuclear deterrent while the threat of US nuclear attack remains.

Until 1991 the US maintained nuclear weapons in South Korea, and the country is still within the wider US nuclear umbrella

"Since the Korean War, the US has maintained a policy of protecting South Korea with its nuclear weapons, and the potential of a pre-emptive nuclear strike against North Korea," said Wol-san Liem, Director of International and Korean Peninsula Affairs at the Korean Federation of Public Services and Transportation Workers' Unions.

"On top of that, on a regular basis the US and South Korea conduct military exercises which are about toppling the North Korean government."

Pyongyang has long viewed those exercises as a direct threat, and Liem added that "all of that militarization, the hostile policy towards North Korea and the fact that there is still a state of war, is part of a vicious cycle."

"We have to figure out how to stop the vicious cycle, but it can't be one sided," she said. Failing to go through any "trust building steps" with North Korea could result in everything falling apart, she warned.