North Korea has had their allies China and USSR/Russia literally next door to help them if needed, they share borders with them. Why do they have long lasting f

North Korea has had their allies China and USSR/Russia literally next door to help them if needed, they share borders with them. Why do they have long lasting famine because of US sanctions while China and Russia/USSR are massive producers of food? Cuban food shortages are a bit more understandable because they're in the western hemisphere surrounded mostly by countries friendly to the US, but North Korea?