Most political scientists, historians, policy advisors and other professionals in politics (or geopolitics) and foreign relations see the U.S. national security policy and diplomacy following in a liberal or realist tradition. Define liberalism and discuss the historical background of the theoretical viewpoint. Also, examine the latest National Security Strategy (President Trump's 2017 NSS), assess the realist and liberal principles reflected in the document. Then compare it the current NSS to the 2015 NSS (signed by President Obama), what principles differ?