EU needs stable alliance to ensure that it has efficient ability to respond anywhere in EU. But it is then question of should EU take additional steps to become federation and form its own military or should member states form EU wide defense alliance that works separately from EU. This isn't new question but something that has been talked even before EU was founded. NATO has kept this question as hypothetical for now, but now it looks that EU needs to make its mind on this issue. Although currently separate alliance is option that can be done because EU currently isn't able to change its constitution. Currently NATO can't function without USA, as majority of its command and intelligence structure is in hands of US. So if USA leaves NATO there is anyway need to reform the alliance and how it is structured in such extend that it will be almost new alliance even if it is still called NATO.