Nation should be able to defend themselves rather than relying on a federation of other states. If a nation joins into a agreement with others, they lose part of their sovereignty and their ability to decide policies for themselves. Nations should be independent in order to protect themselves and their security.
Federation is attuned to allowing the federal government to handle every aspect of national security. National security is mandated for the United Nations. Any segmentation of the United Nations or the National Security Agency is not good for the protection of the American people. Therefore federation is not a solution.