Pyongyang has conducted another record-breaking string of sanctions-defying launches this year, including test-firing the country's first solid-fuel ballistic missile this month -- a key technical breakthrough for Kim Jong Un's military.
In response, Yoon has pulled South Korea closer to long-standing ally Washington, and even sought to bury the hatchet with former colonial power Japan in a bid to contain North Korea.
But the South Korean president has seen his domestic approval ratings dive, hit hard by public disapproval over his handling of a recent US intelligence leak that appeared to reveal Washington was spying on Seoul.
He is also struggling to reassure the South's increasingly nervous public about the US commitment to so-called extended deterrence, where US assets -- including nuclear weapons -- serve to prevent attacks on allies.
A majority of South Koreans now believe the country should develop its own nuclear weapons, multiple surveys show. Yoon has hinted Seoul could pursue this option.
All this "reflects not only growing concerns regarding nuclear North Korea but also an erosion of trust in the US security umbrella", Gi-Wook Shin, a Korea expert and sociology professor at Stanford University, told AFP.
Shin said Yoon was one of South Korea's "most pro-alliance" presidents, "but recent events such as the allegations about the US spying on South Korean officials have not helped him gain domestic support".
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