"World events have led some observers, starting in late 2013, to conclude that the international security environment has undergone a shift from the familiar post-Cold War era of the past 20 to 25 years, also sometimes known as the unipolar moment (with the United States as the unipolar power), to a new and different situation that features, among other things, renewed great power competition with China and Russia and challenges by these two countries and others to elements of the U.S.-led international order that has operated since World War II.
A previous change in the international security environment—the shift in the late 1980s and early
1990s 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. Many of these changes were
articulated in the 1993 Bottom-Up Review (BUR), a reassessment of U.S. defense plans and
programs whose very name conveyed the fundamental nature of the reexamination that had