All right Zig Forums, here are my two cents on the whole EU-army thing.
In theory it's a great idea. Hear me the fuck out though.
Why is it a great idea?
Many European nations are facing the same problems right now: Threat of Russian invasion and impotence on a global scale. Take Poland as an example. The Poland and France as examples. The Poles are very much afraid of getting invaded by Russia in order to connect Königsberg with Russia through Belarus as a first stage of attacking into eastern Europe.
France on the other hand is seeking to strengthen their seat on the strategic stage of the world and their role in their former colonies. Both might seem like goals in opposite directions at first.
One nation needs an army that can fight off a full scale invasion, the other is looking to bomb some huts in the deserts of Africa.
However what if both nations were to work together? France's military industry is on the brink of collapse. Their last gun manufacturer closed down and their (extremely advanced) missile industry is collapsing as we speak. Why? The French government and neighboring nations haven't really had a reason to buy large quantities of those kinds of weapons. You don't need ATGMs to win against unarmoured opponents. However you do need those to equip an army capable of defending a nation.
If, for example, the French were to equip parts of their army to defend Poland, and Poland were to send some of their troops to assist the French in places such as Mali both would benefit. French arms industry would get more contracts and survive. French get aided in killing locals, Poles get some experience with modern firefights and French troops on their borders.
The problems I see with it are crippling however.
a) Who is in charge of this new army?
Which person decides who does what when SHTF. You can't have military decisions done by some EU parliament. The endless bickering over who should be in command would probably stop the project before it got started.
If that one got sorted though b) what if a nation refuses to aid once SHTF?
Do you rip control of their armed forces from them? That never goes well. Do you add a veto right for each nation to refuse taking part in the war? That would make the entire idea useless. Do you impose sanctions? Not gonna work either.
But let's say that all nations always agree to defend their parterns. Then c) LANGUAGE.
Now I am not trying to insult soldiers, but in the EU there are Spanish, French, German (all 24 local accents of it), Polish, Czech, Romanian, Hungarian, Italian, Lithuanian, Estonian, and many many more languages. What language will be the standard. French, English, or German? Will you force all soldiers to learn that one language? Even NATO uses French and English as the official languages. So what happens when the troops on the ground have to cooperate? Will you make some German Grenadier talk to a Hungarian tanker in English? Again, not trying to insult anyone, but infantry doesn't tend to be the best educated social class in any society. Their English will be shit. And they will try to talk to another non-native English speaker in a potentially very loud and uncomfortable situation.
d) NATO
Will all EU army members be forced to leave NATO? What if a NATO member state gets attacked, so almost all EU army members go to help them, but then the EU gets attacked? Surely you would have to prioritize one over the other.
e) outside nations
What if one member state A has a contract with a non member state B and then another member state C attacks the non-member state B. Should A be allowed to defend B against C although both A and C are members of the EU army?
All of these are problems that have been solved before. NATO solved them expertly. The only problem with NATO is the dominant position of the US, and guess what: the only reason the US got that position was because most NATO member states simply decided to shrink their military past a critical point. The US is right in demanding more from their allies. If they want to talk at the bigboy table they must smoke cigars like thee big boys.