As for me, Union is the best option for the european nations. Remember, Immanuel Kant wrote about two options of the future for mankind. A bad option is to be each nation apart, fall lonely or fight each other due to mess. It'll lead to the vast grave of humanity. And a good one is to unite nations into the one union for common help and wealth. That is what Kant called perpetual peace. There will be peace under the both options: with us and without. Though if we want to survive, we should choose a good one. Because of that, European Union as a unifying factor is one of the guarantees of human survival in future perpetual peace. IMO, I'd love more Union of Europe and the World than the isolation and real possibilities of protracted wars.
Developed from the work of Immanuel Kant, the theory of democratic peace simply states that democracies do not fight with each other (Perpetual peace, 1975).
Democratic peace theory:
Monadic theory: Democracies are peaceful and less likely to go to war with any state
Dyadic theory: Argues that democracies are peacefull with each other but are likely to fight with other non-democracies.
Systematic theory: Holds the argument that the international system becomes more peaceful with the increase of democratic states.
Isn't the European Union rather antidemocratic and therefore not an improvement to perpetual peace based on the arguments mentioned above?
It is very not clear, how from the theory of democratic peace comes that EU is antidemocratic and therefore denies a perpetual peace.
First I want to understand what your is point about democratic peace theory. You said that democratic theory of peace claims that democracies either don't fight any state (monadic theory) or don't fight each other, but can fight non-democracies (dyadic theory) - we can say, strong pacifism (when democracies tend to avoid wars) and weak pacifism (when democracies can go to ar with non-democracies). Independently from version of pacifism, the democracies growth makes international system more peaceful and secure (systematic theory).
Are there any facts on the EU or statements in its documents, which tell that EU is antidemocratic and incompatible with perpetual peace? Let's see. First of all, is the EU denies principles of democracy and therefore is antidemocratic. By its statements, the EU is democracy. But as for examples. Do we see persecutions of eurosceptics or EU/NATO tanks in London due to Brexit? No, we don't. Which is not true for the USSR (sometimes people incorrectly equalize the EU and the USSR): there were persecutions of anti-Soviets and the Warsaw Pact tanks in Budapest and later in Prague. Next, can the EU go to war? I can, so it is not strong pacifism. But does not approve the war. War is a last resort, it is ultima ratio regum for Europe. The EU could not tolerate anti-democracies, but going to war with them is the last resort for the EU. And the Union welcomes worldwide growth of democracy. So it could very undesirable contradict to the monadic theory, it doesn't contradict to the dyadic theory (there was no war between the EU members inside) and systematic theory.
So the EU is democratic and compatible with perpetual peace since I do not see the opposite reasons.