The world today is undergoing major changes unprecedented in a century，and the major strategic forces are speeding up the adjustment of their international strategies to adapt to the ever-changing international environment. There have been some consensuses on their perceptions of the changing international security environment，such as a more complex security environment，a more intense strategic game，the weakening and divergence of Western power，and a more pronounced multipolar trend，among others. However，there are differences in their strategic goals and policy responses：the U.S. is committed to maintaining its global dominance，by way of alternating between multilateralism and unilateralism and using a two-pronged approach of tactical contraction and strategic offense；Russia is making use of its strategic advantages and reorienting its diplomacy，trying to continue to exercise unique influence in international affairs and maintain its strategic position as an important power；the EU，facing the challenges of power crisis，identity crisis and value crisis，tries to achieve a balance between strategic autonomy and maintenance of Atlantic relations，so as to participate in global governance as a significant international force.