The Korean Demilitarized Zone with South Korea remains the most heavily fortified border in the world. Inter-Korean relations are at the core of North Korean diplomacy and have seen numerous shifts in the last few decades. North Korea's policy is to seek reunification without what it sees as outside interference, through a federal structure retaining each side's leadership and systems. In 1972, the two Koreas agreed in principle to achieve reunification through peaceful means and without foreign interference. On 10 October 1980, then North Korean leader Kim Il-sung proposed a federation between North and South Korea named the Democratic Federal Republic of Korea in which the respective political systems would initially remain. However, relations remained cool well until the early 1990s, with a brief period in the early 1980s when North Korea offered to provide flood relief to its southern neighbor. Although the offer was initially welcomed, talks over how to deliver the relief goods broke down and none of the promised aid ever crossed the border.