My arrival in South Korea on Feb. 10 for the Pyeongchang Winter Olympic opening ceremonies was jarring.

I had just come from Washington, where the perception of North Korea as both a serious threat to U.S. security and as a barbarous dictatorship is deepening.

Transiting from Incheon airport to the sleek KTX bullet train, en route to Pyeongchang -- itself little more than 80 kilometers from the demilitarized zone, I found South Korean travelers clustered around TV sets, raptly watching a North Korean symphony concert, and absorbing news that Kim Jong Un's younger sister had also just arrived for the Olympics hours before.

Gangwon Province, where the Olympics are being held, directly borders on the demilitarized zone, and hosts 200,000 South Korean troops -- one third of the country's entire land forces. Yet when I passed through the gate at Olympic Stadium there, one hour early, I competed futilely for attention with a comely North Korean cheerleading squad 230 members strong, stylishly yet informally clad in their red warmup suits, hustled rapidly through security just ahead of me, to a conspicuous yet secluded upper corner of the arena.

There they cheered rapturously for the Korean taekwondo demonstration and even more vigorously when the unified North-South "Team Korea" entered the stadium.

The cheering squad had good reason for its energy -- Kim Yo Jong and North Korea's formal President Kim Yong Nam, not to mention U.S. Vice President Mike Pence, Japanese Prime Minister Shinzo Abe, and South Korean President Moon Jae In, were all watching from the VIP reviewing booth directly before them.

The formal side of the opening ceremonies stressed a theme of reconciliation again and again. International Olympic Committee President Thomas Bach declared that "These Games...are inviting the world to join in a celebration of hope." U.N. Secretary General Antonio Guterres echoed those sentiments.

South Korean President Moon and his wife, although they stood physically side by side with the Pence couple, made a point of greeting the North Koreans, seated directly behind them.

And the musical menu amplified a peace through global collaboration theme -- the idealistic message of John Lennon's "Imagine" ("...keep on dreamin' until the world is one") was followed by a lyrical admonition to "Let everyone shine."

Pence and Abe -- sitting side by side -- were having none of this. Pence brought the father of Otto Warmbier, the American student brutalized and fatally injured in North Korea for stealing a poster, as a guest to the opening ceremonies, and met with North Korean defectors immediately beforehand.

He pointedly avoided the North Korea VIPs, making an unprecedented visit to the South, even though they were seated 2 meters away, and did not stand when the unified North and South Korean team entered the stadium. Abe responded similarly to the North's presence, although he did meet briefly with Kim Yong Nam at a broad diplomatic event.

The pointed U.S. and Japanese diplomatic coolness has clear origins in national interest, with simple short-term stability on the Korean Peninsula, in the face of steadily rising North Korean nuclear and missile capabilities, increasingly unpalatable.

Both Washington and Tokyo naturally see those rising North Korean capabilities as a serious threat. Japan could well be the first target of North Korean weaponry, in the view of many specialists, even though South Korea is much closer geographically to the North. Recent ICBM tests by Pyongyang indicate that most of the United States will soon be in range of North Korean missiles as well.

Beyond the direct military challenge that North Korea could pose to Japan and the United States is the sobering potential impact on America's alliances. The traditional basis of those alliances has been unassailable relative American military strength.

If North Korean capabilities begin forcing the United States to render Portland vulnerable should the United States need to militarily challenge Pyongyang, the obvious difficulty of such a decision for Washington could begin to erode Japanese and possibly South Korean confidence in the U.S. nuclear deterrent itself, producing new and potentially destabilizing hedging strategies on the part of Tokyo and Seoul.

For Seoul, the dangers of a Northeast Asian geopolitical equation featuring North Korea as a rising, more militarily potent power center could potentially be offset if North and South made common cause, as Pyongyang is currently hoping.

For Japan, there is no such silver lining. Tokyo, as Korea's longtime colonial master, cannot easily present itself as anything but a political adversary and strategic threat to Pyongyang. Like the United States, Japan remains a principal adversary of the North, under virtually any scenario.

The stronger, more accurate, and longer-range that North Korean capabilities become, the more the shadows of a strategic divide between the United States and Japan, on the one hand, and their ally South Korea begin to deepen.

Pyongyang, of course, has incentives to play intensely on this prospect in every way that it can. Its missile tests, its nuclear development, and its charm offensive all seem directed at co-opting Seoul, to the disadvantage of Tokyo and Washington.

Time is of the essence in changing North Korea's direction, since Pyongyang's capabilities are rapidly advancing. The United States and Japan thus inevitably are adopting a high-pressure strategy unlikely to relent until North Korea agrees to denuclearization.

The long-term issue is not simply peace on the Korean Peninsula, in the abstract, but the broader strategic configuration of the Northeast Asian region, as well as the potential of Pyongyang to exert destabilizing leverage on the credibility of longstanding alliances.

Effective U.S.-Japan pressure tactics, however, must bring South Korea along politically -- a complex task, in view of post-Park politics in the South, not to mention still smoldering historical issues between Japan and Korea. More South Koreans than not, mistrusting Pyongyang's intentions, still favor pressure to unilateral conciliation.

Yet Pyongyang's charm offensive is playing intensely on the heart strings of Korean identity, throughout these Olympics. Both Washington and Tokyo need the expertise of diplomatic veterans who know all three side of the strategic and changing U.S.-Japan-Korea triangle to deal with this subtle seduction. They need to keep up the pressure, but also to understand and respond to the side effects in Seoul.

The Korean Peninsula thus remains a political battlefield throughout these Olympics, just as surely as it was a bloody military battlefield 65 years ago.

(Kent E. Calder is director of the Reischauer Center for East Asian Studies at SAIS/Johns Hopkins University in Washington.)