Cost, Policing Effectiveness, End Goal, and the Social Contract of a Country
I think it's interesting to look at the reasons why politicians make such statements of need.
Most of us get to elect our politicians, some of us pay some tax, and then we hold them to account for their response to occasions when bad shit happens. Look at Spain - they turfed out the ruling party at the general election soon after the Madrid train bombings.
Understandably then politicians are keen (well, they should be keen) to do a half-decent job of preventative policing. Using only restorative policing is unlikely to result in plaudits after a bad incident; the incident has already happened.
Cost, Morals and Feasibility of Surveillance
But preventative policing costs money, and is much more elusive than restorative policing. For the Police to be able to guarantee preventing someone doing something nasty they basically have to follow them all the time, with the aim of intervening as soon as their intentions become clear. And the only way for the police to guarantee on their own that no-one will be able to launch an attack is for them to follow everybody, all the time.
However, as well as being pretty morally bad, it's not affordable to do that for everyone all the time. East Germany tried to do this, they used some pretty nasty methods to force wives to spy on husbands, etc, with a great saving to state expenditure (even communist governments had to pay their own spies).
Laws, Borders between Policing Zones
The right laws can help. It's useful to be able to pin a charge on someone that'll send them to jail before they let off their big bomb, shoot up a cinema, whatever. If there's laws that make it an offense to prepare an attack, then there's a route open for policing intervention well in advance of the actual attack. There's no need to take it down to the wire before intervening. If they're in jail having been convicted of such an offense, they then cannot carry out their plan.
In the USA they're in very poor shape in this regard; more or less anyone can legally possess a severely dangerous weapon, and they've not broken any law right up until they pull the trigger whilst aiming it at someone, by which time it's too late. Gun control is pretty lax in a lot of Europe, so similar problems there. Gun control in Britain is pretty tight, so anyone found in possession of a weapon most likely has broken some law or other, so they can be charged, tried and jailed if convicted, all before they've pulled that trigger. Same for knives, diesel / fertiliser, pamplets of a certain sort, etc.
Being an island helps too; there's less opportunity for a miscreant to exploit slack policing in one country to launch an attack in another. If one has open borders but variations in policing standard / laws / etc, such as is the situation in Europe, then it's easier for a miscreant to succeed.
Closing the borders afterwards just makes them look daft. In fact, despite borders clanging shut all across Europe on occasion in recent years, I've not heard of any discussion within Europe about modifying Schengen to make dealing with such events better thought out.
Any Beneficial Effect from what Germany and France are Calling For?
So one is not going to go down the route of the Stasi / East Germany because it's too expensive or abhorrent. Nor is Europe intending to achieve full political / law / policing union across Europe. Of course, one of Europe's big problems is that it's a terrible splitting of sovereignty. Police forces, which are still the responsibility of each sovereign nation, have no idea who is in their country because there's no borders between them. And if you're not looking for nasty people at one's borders, your only option then is to look for nasty people within, which is always going to be more intrusively Stasi-esque.
How then does the law enforcement types get tip-offs?
Germany and France seem to be trying to do it the lazy way, thinking that it will be possible to solve the problem simply by snooping on everyone's email / facebook / whatever.
Snooping no doubt produces a lot of data. Turning that into accurate and useful information cannot simple, even if one somehow has the magic keys that unlocks everything. Used on its own as a tool for making decisions about people must inevitably be a blunt and inaccurate instrument.
Even if they did achieve that data-haul nirvanah that they're calling for, it would be short-lived as a means of finding out if bad things are about to happen. The canny sort of nasty guys would just stop using electronic communication altogether (as has happened before; the Taliban got wise to how dangerous a mobile could be). Meanwhile the nasty guys would still be grooming impressionable and vulnerable people, they'd stilll be improvising explosive devices, acquiring guns, buying machetes, hijacking lorries, spreading a lot of hate talk, and they'd be using the postal service instead (that's even less surveyable than electronic comms).
I reckon comms surveillance does serve a role - you have to do it because if you don't then comms will (and has been) be used by nasty guys, and it is useful to them. There's no need to make it too easy.
Policing With the Consent of the People
The most effective measures to prevent problems includes a strong and willing relationship between law enforcement and the communities they're there for. People need to feel that if their son or someone they know is going off the rails that getting in touch with the local cops will ultimately be a good thing. They need to know that dibbing in a trouble maker no matter who they are will be a good thing for them an their kin. Good relationships with people yields tremendous results - they'll be keen to make the police aware of problems that are developing.
Best bit - is basically free.
Bad relationships with people cannot be compensated for by technology of any sort, no matter how much of it one has.
Certainly this is one of the lesser mentioned successful aspects of policing in the UK in recent times (people focus a lot on GCHQ and the like), and is a welcome even if it is not yet a universal change from how it used to be in the bad old days. It takes proactive and continual work by police - you know, speaking to people on occassions other than when they're arresting someone, making a point of calling in on local bigwigs for a cup of tea and a chat, etc.
That, backed up only when necessary by strong surveillance and strong application of the law, works. Surveillance on its own does not, because you can never afford to have enough of it to guarantee that the end goal of law-and-order (namely, settled and contented order in a country) is achieved, and it cannot be acted on in isolation without tremendous cock-ups that impact perfectly innocent people bringing the whole thing into disrepute. Altogether now, "Brasil, meu Brasil Brasileiro / Meu mulato inzoneiro / Vou cantar-te nos meus versos"
It's the aspect of policing that had so catastrophically failed in Brussels, Paris, and seemingly large parts of the USA, etc. There should be no such thing as a police no-go zone; it's a curious kind of racism to refuse to police an area. No-go zones mean that the police have been doing it wrong.
So I'm unimpressed by Germany and France's proposal. I'd be more impressed if they (particularly France and Belgium) were talking about doing something about getting policing back to normal in the communities that have been neglected for far too long allowing the nastier types to find a safe haven.