The largest refugee crisis ever witnessed since World WarⅡis taking place in Europe. World attention is focused on the reaction of the EU and its member states，as well as that of the United States who is the greatest ally of the relevant European countries. A series of measures taken by EU notwithstanding，there still exists severe challenges. In this crisis，the Obama administration reacted with great caution，offering limited help to Europe in resettling the refugees. In any case，the refugee crisis would probably last for several years，producing complex and profound influence on the future of the transatlantic alliance.