The EU ensures that all countries that it comprises share a mutual interest. It decreases the incentive towards competition from nations within its circle and fosters an atmosphere of cooperation instead. Since conflict is typically generated by self-interest. The EU has curbed that by getting nations to join forces instead.
I do believe that the European Union is ultimately responsible for its member states security and peace. the main reason the European Union was created was to make the region a more safe and prosperous place for citizens to live. The recent instability in the EU is a global issue but the EU needs to take the lead.
Of course, the EU is responsible for European peace. Who else should be? If this collective of countries wants to be such a union, then it is incumbent upon them to provide for its own peace. There is no reason whatsoever that other countries, such as the U.S. should expend its wares, people and lives to advance a foreign collective like the EU.
The European Union has brought its member countries together under the goal of promoting trade and achieving political unity and understanding within its borders. These goals are admirable. Specifically, the aim of political unity helps the EU settle scores in a diplomatic way. I believe that the EU has accomplished these aims and is therefore responsible for the decades-long era of peace that Europe has now enjoyed.