North and South Korea marched under a united flag at the 2018 Winter Olympics opening ceremony in Pyeongchang on Friday in a symbolic break in tensions between the two nations over North Korea’s nuclear program.

Athletes from the two countries entered the Pyeongchang Olympic stadium together, joined hands, and marched under the Korean Unification flag, which displays the entire Korean peninsula in blue against a white backdrop.

It was a rare but not unprecedented spectacle. The two countries first displayed the flag at the 1991 World Table Tennis Championships and most recently at the 2006 Winter Olympics in Italy.

But this year’s gesture of unity comes at a crucial moment. Tensions between North Korea and South Korea have skyrocketed as Pyongyang’s nuclear and ballistic missile programs have advanced at an astonishing pace in the past year, and North Korea’s leader, Kim Jong Un, has persistently demonstrated a readiness to make use of them.

During his New Year’s Day speech, Kim reached out to South Korea and raised the idea of starting high-level talks for the first time in two years to discuss the possibility of North Korea’s participation in the Olympics.

South Korea eagerly took him up on the offer, and through negotiations at the border village of Panmunjom the two countries quickly agreed not only to march under a united flag but also to form a joint women’s ice hockey team, marking the first time they have contributed athletes to the same team at the Olympics.

North Korea then further pressed its charm offensive, sending Kim’s younger sister, Kim Yo Jong, to the opening ceremony — the first time a member of his immediate family has set foot in South Korea since the Korean war in the mid-20th century. She shook hands with South Korean President Moon Jae-in at the beginning of the ceremony.

Kim Yo Jong also sat just feet away from Vice President Mike Pence. But there’s currently no indication that they exchanged any pleasantries.