

Not saying that’s not a great idea, but just think of what it would entail - for starters, who would be the commander in chief of a European army?
Not a foreign adversary, probably France
The EU needs to remove all us military from it’s territory, replicate any NATO structures as EU ones and even make sure there are no people in our armies with confused loyalties. This needed to happen before Trump even became a candidate for 2016.
Port forwarding is necessary due to NAT not firewalls.
It’s not that your router blocks new incoming connections at port X, it’s that it does not know which local client it’s meant for, since it’s addressed to the public IP that is held by your router.
With IP6 it’s lan client also gets assigned a public IP6 address (as there are plenty) and so the router receives a connection addressed to a Lan client and knows where to route it.