NATO has members who are not in the EU. How can you say NATO is a European defence union Look very closely at the NATO nations. Not many are not EU member states:
'Albania, Canada, Iceland, Norway, Turkey, and the United States, which are members of NATO but not of the EU, participate in all NATO-EU meetings. So do Austria, Finland, Ireland, Sweden, and since 2008, Malta, which are members of the EU and of NATO's Partnership for Peace (PfP) programme.'
Norway has an EEA arrangement with the EU. Iceland has rejected the EU. Albania is an accession country to the EU, as is Turkey.
It's also enshrined in the North Atlantic Treaty that founded NATO:
Article 5
The Parties agree that an armed attack against one or more of them in Europe or North America shall be considered an attack against them all and consequently they agree that, if such an armed attack occurs, each of them, in exercise of the right of individual or collective self-defence recognised by Article 51 of the Charter of the United Nations, will assist the Party or Parties so attacked by taking forthwith, individually and in concert with the other Parties, such action as it deems necessary, including the use of armed force, to restore and maintain the security of the North Atlantic area.
Any such armed attack and all measures taken as a result thereof shall immediately be reported to the Security Council. Such measures shall be terminated when the Security Council has taken the measures necessary to restore and maintain international peace and security .
Article 6 1
For the purpose of Article 5, an armed attack on one or more of the Parties is deemed to include an armed attack:
<strong>on the territory of any of the Parties in Europe</strong> or North America, on the Algerian Departments of France 2, on the territory of or on the Islands under the jurisdiction of any of the Parties in the North Atlantic area north of the Tropic of Cancer;
<strong>on the forces, vessels, or aircraft of any of the Parties, when in or over these territories or any other area in Europe in which occupation forces of any of the Parties were stationed on the date when the Treaty entered into force or the Mediterranean Sea</strong> or the North Atlantic area north of the Tropic of Cancer.
Hope that clarifies it for you.