Influenced by the accelerated Russia-Ukraine conflicts，the extremists’ violent terrorism attacks and Brexit，the European Union has unprecedentedly incorporated its common security and defense policy into the “fast-track”of decision-making at the European level. This article argues that under the guidance of the “multi-speed” idea，the Permanent Structured Cooperation（PESCO）mechanism approved at the end of 2017 is characterized with a pragmatism which doesn't require all member states’ participation with military cooperation based on a “module”style. However，the fate of PESCO will be seriously threatened by the factors such as the fragmented defense industry structures and differentiated economic interests among the member states，as well as the on-going tensions between the EU，the U.S. and the NATO. Meantime，the transatlantic relation is facing challenges from Europe's dream of a defense union.