South Korean President Yoon Suk-yeol and his U.S. counterpart Joe Biden will celebrate 70 years of their countries’ alliance at their summit in Washington on Wednesday, but amid all the pomp, the two leaders face difficult conversations about the U.S. commitment to fighting a nuclear North Korea.

Despite efforts to bolster joint military capabilities over the last year, concern is growing in Seoul about the DPRK’s rapidly advancing weapons program.