The beginning of the 21st century was marked by the growing popularity of the idea that flexible military-political coalitions built for concrete tactical tasks had certain advantages over long-term strategic alliances. This strategy was actively pursued on the international stage by the George W. Bush administration during its first term (largely owing to Defense Secretary Donald Rumsfeld's efforts) due to mounting contradictions caused by the transformation of the world order and the changing place and role of the United States in it. The U.S. gave up the concept of collective approval for external interference and stepped up unilateral actions. As existing collective security institutions become increasingly plagued with problems, flexible coalition strategies gain more popularity as evidenced not only by U.S. actions but also by those of Russia. While being an important element of the modern world order, which is currently in transition from a unipolar to a multipolar (polycentric) model, the strategy of flexible coalitions has its limits, the most important of which is their destructive impact on the development of international relations in the absence of all-encompassing collective security structures (global and/or bloc).