The nations close to the Korean Peninsula are:
China: Shares a land border with North Korea and has a maritime border with both North and South Korea.
Russia: Shares a land border with North Korea.
Japan: Is located across the Sea of Japan and has a maritime border with both North and South Korea.
South Korea: Located on the southern half of the Korean Peninsula.
North Korea: Located on the northern half of the Korean Peninsula.
These five countries are the ones closest to the Korean Peninsula.