National security -- United States United States -- Military policy 15 Terrorism -- United States -- Prevention 13 Security, International 11 Civil defense -- United States 7 United States -- Defenses 7 World politics -- 21st century 7 more ... Intelligence service -- United States 5 Internal security -- United States 5 Emergency management -- United States 4 Export controls -- United States 4 National security -- Europe 4 Terrorism -- Prevention -- Government policy -- United States 4 Terrorism -- United States 4 United States -- Foreign relations -- 1989- 4 United States. -- Army 4 Military research -- United States 3 National security 3 Terrorism -- Prevention -- United States 3 United States -- Foreign relations -- 1945-1989 3 United States -- Foreign relations -- 2001-2009 3 United States. -- Dept. of Homeland Security 3 War on Terrorism, 2001-2009 3 World politics -- 1995-2005 3 Combat survivability (Military engineering) 2 Global commons 2 National security -- Canada 2 National security -- European Union countries 2 National security -- Turkey 2 Nuclear weapons -- Government policy -- United States 2 less ...