Can't we have national armies with the main goal of protecting each nation from threats while simultaneously having an overarching European army that can provide reinforcements to each region that needs it?
Lots of EU and European countries already have types of coalitions and military cooperations aimed at defending common strategic interests e.g. NB8 or the Joint Expeditionary Force. These could be the main forces protecting exactly what they are meant to protect, but with a combined European army on top of that, reinforcements could be provided when needed.
Is this already the idea behind an EU army? I have a feeling many people think such a concept would nullify each nation's own defense capabilities.