1. The most intense event in terms of national interest is obviously the 9/11 attack on the World Trade Center and Pentagon. There are several explanations that place this event on top of the list. First of all, this is the first significant attack on US soil. All the other conflicts that the US has been involved in during its history have been fought on foreign ground, but this was an attack in the heart of New York and of Washington. Second of all, the enemy in this case is much more elusive than in the other examples, where it's much easier to identify the opponent, a certified enemy. With terrorists, it's difficult to identify who they are, where they are hiding and, especially, what are the appropriate and efficient instruments to fight them.
             The next level should be the Japanese attack on Pearl Harbor. This can be partially assimilated with an attack on US soil, but the fact that it is distant from the mainland makes it less 'intense' than the previous one. It is, however, a direct and active threat against the US, very real and with deep implications for the subsequent chain of events.
             The Korean War should probably come next, but on the same level as World War II. The implications of the Korean War and its similitude to the world war are given by the ideological battle that was at stake here. More so, Communism is during the 1950s in a period of full expansion, so, despite being an indirect threat to the US security, it is nevertheless something very actual, very present and a threat to be reckoned with.
             World War II is intense through the scale of combat and the dimension of evil (including the Holocaust and general persecutions that come along). However, for the US, it never is a direct threat. Germany was permanently committed to dominating the European continent rather than the entire world, while Japan looked only to the achievement of a large empire in the Pacific. It is more likely ...