World events since late 2013 have led some observers to conclude that the international security environment is undergoing a shift from the familiar post-Cold War era of the last 20-25 years, also sometimes known as the unipolar moment (with the United States as the unipolar power), to a new and different strategic situation that features, among other things, renewed great power competition and challenges to elements of the U.S.-led international order that has operated since World War II. A shift in the international security environment could have significant implications for U.S. defense plans and programs. A previous shift in the international security environment-from the Cold War to the post-Cold War era-prompted a broad reassessment by the Department of Defense (DOD) and Congress of defense funding levels, strategy, and missions that led to numerous changes in DOD plans and programs. A new shift in the international security environment could similarly have a number of implications for U.S. defense plans and programs. Of perhaps the greatest potential significance, such a shift could lead to a change in the current overall terms of debate over U.S. defense plans and programs. Russia's seizure and annexation of Crimea, as well as subsequent Russian actions in eastern Ukraine and elsewhere in Eastern Europe, have already led to a renewed focus among policy makers on maintaining and strengthening U.S. and NATO military capabilities in Europe, and on how to counter Russia's so-called "ambiguous warfare" tactics. China's actions in the East and South China Seas have prompted a focus among policy makers on how to counter China's so-called "salami-slicing" tactics in those areas. A shift in the international security environment may also be generating implications for areas such as nuclear weapons, submarines and antisubmarine warfare, and DOD reliance on Russian-made components.