1. The creation of the EU army was blocked by the UK (saying that an EU army would be a rival to NATO) but since the UK is leaving, the full blood EU army is free to go.
2. There is already EU rapid reaction force, Eurocorps, consisting of several EU battlegroups, under the command of the EU Council.
3. 22 of 27 EU countries are in NATO, so there is already a military cooperation between the vast majority of EU countries in place.
4. Article 42 of the European Union Treaty already says that if some [EU] member is a victim of armed aggression, other EU members have an obligation to help with any possible means.
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So far, everything is prepared, the only thing [yet] missing is a central command, which is being blocked by the UK. But the UK is leaving the EU in a near future.
https://eeas.europa.eu/headquarters/headQuarters-homepage/33557/eu-battlegroups_en
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Germany and the Czech Republic are already building the core of the EU army - some Czech and Romanian special forces are being integrated into the German army:
[video=youtube;hajJ_Z5nF14]