To say that the United States has pursued diplomacy with North Korea is a little bit misleading. It did under the Clinton administration, though neither side completely lived up to their obligations. Clinton didn't do what was promised, nor did North Korea, but they were making progress. So when Bush came into the presidency, North Korea had enough uranium or plutonium for maybe one or two bombs, but then very limited missile capacity. During the Bush years it's exploded. The reason is, he immediately canceled the diplomacy and he's pretty much blocked it ever since.