A European army is a hypothetical army of the European Union that would supersede the Common Security and Defence Policy and would go beyond the proposed European Defence Union. Since no such unified army is currently established, defence is a matter for the member states individually. The member states are, however, bound by several obligations arising from international treaties, and they conduct several multinational initiatives.

The idea of a European army was first discussed in the 1950s. It was proposed by France and would have consisted of the "inner six" countries (Belgium, France, Italy, Luxembourg, the Netherlands, and West Germany), in order to strengthen defence against the Soviet threat without directly rearming Germany in the wake of World War II. In 1952 the Treaty establishing the European Defence Community was signed but not ratified by the signatories.

However, during the Cold War, Western Europe relied on NATO for defence, precluding the development of European cooperation. Immediately after the "fall of communism", the defence apparatus was preoccupied by NATO expansion into the former Soviet bloc. The idea of a European army gained popularity after the September 11 attacks and NATO's involvement in conflicts outside of Europe. In a phenomenon dubbed diversification of European security, NATO has come to be responsible for "hard" threats while the European Union has taken a greater role in "soft" threats, including peacekeeping in the western Balkans. The 2009 Treaty of Lisbon also has furthered defence integration within the EU. This has led to support for a European Defence Union, which would be a step higher in collaboration than the current Common Security and Defence Policy.

In 2019, Germany and the Netherlands activated 414 Tank Battalion, the first that included soldiers from two EU countries. The battalion was created because Germany did not have enough soldiers, while the Netherlands lacked tank capability. This was described as a step towards a European army. However, the experience of the Franco-German Brigade in Alsace has been different, facing challenges due to greater linguistic and cultural differences. Notably, Germany's "innate caution since World War II about military intervention."

Under the current arrangement, there is no EU army and defence is reserved for the member states.

According to current Treaty on European Union article 42(7):

If a Member State is the victim of armed aggression on its territory, the other Member States shall have towards it an obligation of aid and assistance by all the means in their power, in accordance with Article 51 of the United Nations Charter. This shall not prejudice the specific character of the security and defence policy of certain Member States.

According to the legal opinion prepared for the European Parliament, such obligation is stronger than NATO Article 5:^{[quote needs citation]}