Homeland Security Digital Archive - It provides quick access to important U.S. policy documents, presidential directives, and national strategy documents as well as specialized resources such as theses and reports from various universities, organizations and local and state agencies.
Military Balance - Main Library UA 16 .L65 (latest edition in Reference) - detailed data on all things military from the International Institute for Strategic Studies.
Proquest Congressional - great searching tool for everything Congressional including the very useful Congressional Research Service Briefs. You can limit your search to just CRS reports which give great background on current issues being debated in Congress.
SIPRI Yearbook - Main Library UA 10 .S7 - (latest edition in Reference) - from the Stockholm International Peace Research Institute annual compilation of data and background on armaments, military, and peace efforts.
U.S. War College Guide to National Security Issues - most recent edition 2010, downloadable 387 page pdf
Center for Security Studies - open source digital library of reports, studies, etc. from think tanks, research institutes and NGOs from around the globe
Intelligence Resource Program - debates, hearings, and reports from Congressional intelligence committees from 1989-2012 compiled by Federation of American Scientists.
International Crisis Group - think tank specializing in national security issues
International Institute for Strategic Studies - UK based well respected think tank focusing on security issues
Terrorism - Law Library of Congress Research Guide to the topic and legal issues surrounding it