One idea would be for the EU, instead of individual member states, to stand up a professional army. Then you don't have to force people and it also just might bind European countries more closely together.
A joint European army does nothing to reduce US leverage; the US is still the only meaningful nuclear deterrent to Russia, and everyone in Europe knows it. But if the EU is able to credibly counter conventional threats, that's a huge bonus for the US.
While the UK and France have nuclear warheads, their numbers (225 and 290, respectively), pale in comparison with that of Russia (5889) and the US (5244).
Also, the UK's force is sea based and France's is land and air based, both Russia and the US have a nuclear triad:
three-pronged military force structure that consists of land-launched nuclear missiles, nuclear-missile-armed submarines, and strategic aircraft with nuclear bombs and missiles.[1] Specifically, these components are land-based intercontinental ballistic missiles (ICBMs), submarine-launched ballistic missiles (SLBMs), and strategic bombers. The purpose of having this three-branched nuclear capability is to significantly reduce the possibility that an enemy could destroy all of a nation's nuclear forces in a first-strike attack. This, in turn, ensures a credible threat of a second strike, and thus increases a nation's nuclear deterrence.
https://en.wikipedia.org/wiki/List_of_states_with_nuclear_we...
Around 90% of the non-rural Russian population is in their most populous 100 cities. Taking those out would pretty effectively destroy Russia as a country.
Even if the UK managed to launch successfully without those systems being taken down (and there will be a missile failure rate), Russia has sophisticated air defenses, and there will be a substantial loss rate. Only a small fraction of those warheads will hit.
And those are not megaton warheads. These are small nuclear weapons meant for battlefield use. What's more, the UK is simply not going to go for a genocide; they're going to hit military assets, which are far more spread out.
So while the UK can deter from conventional attacks on itself, it's by no means a MAD scenario. They do not really have a shield to extend across eastern europe the way the US does.
I am all for a professional EU army but forcing - let's face it - young men to join, even for an year, doesn't sit well with me.
There are some calls for bringing back military service in some countries. I'm not sure that's needed or a good idea. The reason most European armies are not in great shape is not a lack of people but years of budget cuts and mismanagement. That would be because they don't actually do a whole lot of fighting normally. They defend without ever being attacked and there is the odd mission abroad where some countries send maybe thousands of troops at best. I don't see that change any time soon. And that's a good thing. I count myself lucky to not have been exposed to major wars close to where I live. And I live in the middle of Berlin. Buildings here still have scars from WW II. At least the few that survived that.
The most credible threat on our eastern border was the Russian army. And courtesy of their own ineptness, they seem to be hard at work reducing that threat by expending most of their officers and usable equipment in the Ukraine. It doesn't look like they are recovering soon.
Also, there is the point of this being a remarkably old school conflict. The Russians tactics have barely evolved since the 1940s. They seem unable to use their airforce effectively, their navy is not a factor either. Mostly they are just launching wave after wave of cannon fodder at their enemies. Brutal and it's not working great for them judging from the recent news.
Most modern conflicts are very different. Relatively poorly trained conscripts don't add a lot of value in such conflicts. Europe needs lots of rockets, drones, etc. Keep the fight far away from its borders.
A bigger risk would be an armed uprise within our borders. For that reason getting a lot of small countries excited about building out their armies is not a great idea. I'm old enough to remember the Bosnian war and I've visited Mostar a few years ago. Political populism and armies are not a great mix. A lot of blood got spilled in Europe that proves that.
On the other hand, it could get very awkward if we had another situation like we had in 2002/2003 when the US were asking other countries for support in their war on Iraq. Europe was deeply divided - at least at the leadership level - in this matter. With a unified EU military, countries would have to give up a large amount of sovereignty. And there is already a fair amount of hostility towards the EU for taking sovereignty away from member states, remember Brexit.
Personally, I think it would be a good idea if all countries could agree to give up their own national military. But I don't think it will happen. At most, I could see countries agreeing to move more authority to the NATO.
Last but not least, there are more than 25 languages being spoken in the EU. We'd have to agree on one language being the lingua franca in a unified military if we want it to work.
If you can't disagree and commit, how unified are you really?
From a cursory read, it doesn't seem like that stands in the way: https://en.wikipedia.org/wiki/European_army
In some remarkable circumstance that the EC proposed such a thing, and all the MEPs did think that was a good idea, there'd still have to be an EU treaty that allowed for it, because it's certainly beyond the scope of the CFSP. Said treaty would have to be ratified by the member states.
If the EU wants to have an army that serves their interests, I would consider it prudent to have the command of that army be in EU hands, instead of just handing their soldiers to the US. They can still join defensive treaties with the UK, or simply stay in NATO, but that's a different question from how the army is organized when Article 5 of NATO hasn't been triggered.
If we are to convince France to merge their army with the common EU force the EU might be forced to also protect French interests in West Africa (they are quite involved in that region still)
That is, its members transfer part of their sovereignty to the EU
Theoretically this can change at any time with the agreement of the members.
That said: even now the UK has left, there are many in the EU who don't want total integration — as you can see from other aspects like the Euro, Schengen, and the Blue Card not applying to all member states — combined with with the language barriers between almost every pair of states[0], I think this would be a long time coming even if everyone was on board.
Although, NATO is a thing and arguably does all that without any of those problems, so perhaps it's easier than I'm assuming…
[0] Austria and Germany being an obvious exception, except they're banned by treaty from ever being combined again