Today is the day of German unity, a national holiday in reference to the long periods without a unified national state. (Almost nobody -and nobody official- still considers Austria or East Germany east of the Oder as "German", a distinct change to the state of affairs 150 years ago).
This is probably a good time to write about political unity (with some token reference to defence). The project of European unification began during the 1920's as German-French reconciliation talks which ultimately failed because France didn't want to forfeit its reparations claims and Germany changed governments rather quickly at that time. The next approach during the 1950's worked, in part because de Gaulle in his own way heeded the wisdom of having friends close and foes closer. The first attempts at cooperation and common handling of issues later grew to a real European unification project.
European Unity took a monumental step to succeed; it had to move from pragmatic, advantage-driven policy towards an ideology. Ideologues can easily muster a lot of enthusiasm and a lot of tolerance for shortcomings of the own ideology in action, and were thus ideal cheerleaders for the project. Sadly, ideologues also bring a long a lot of simplistic thinking (such as unification = always good) and an intolerance for steps backward (even if a mistake was made, which of course they wouldn't recognize).
It all went well for a long time, and the European Unification behaved like a bureaucracy does in theory; it grew as much as possible, but always intent on adding more benefits than costs. There was no cost/benefit or "cost minus benefit" optimisation, but legitimacy and majority support was maintained by providing a net benefit at least.
Then came the European Unification step of a common currency, and the ideologues (among them German top politicians) made one gross mistake: They misunderstood "commonality" for "unity". "Commonality" is a subset of "unity", but it doesn't necessarily support the latter. The common currency removed market mechanisms that provided balance and automatic corrective forces, in favour of commonality - misunderstood as unity. The problem is that the removal of these automatic corrective forces (mostly the function of flexible currency exchange rates) did lead to an ever widening gap of imbalances. Europe wasn't uniting -even though a lot of naive people trusted with much of other people's money believed so, too. Europe was and is being divided by the common currency.
Style (commonality) had won over substance (unifying forces). The ideologues are still out there, though. They don't get the birth defect of their creation and decry even thoughts of going a step back from a hell of a mistake.
As a consequence, we're facing another monumental challenge: In order to actually save and propel forward the European unity and thus our best long-term hope for peace in Europe, we need to break the ideology of European Unification. Europe has to move from a style-obsessed and ideology-driven bureaucracy to a common interest-driven project with emphasis on actual benefits again. This will bolster the legitimacy of European unity, and help us to make the next steps.
One of which needs to be that we perceive the world similarly. We simply need European news.
There's not going to be a sense of unity as long as a red line on a map determines whether a scandal about a poorly constructed public building makes it into our news. As of today you can move the building site by only 50 metres across the magic red line on the map and this way decide whether a German or a Frenchman in the respective capital will read about the scandal in his newspaper.
So here's a commonality that might actually foster unity: Common news.
This was actually A LOT about national security, or "war and peace". European unification may turn out to be the thing that eliminates warfare on European soil for decades if not generations to come. It could play out as one of the biggest national security achievements ever. It might also turn towards ugly real quick, though.
The challenges of our time appeared to be to (re)gain full employment, handle the demographic change (if possible to no growth), handle the transition to a sustainable energy economy and such. The Euro currency debacle has added saving the European Unification to this list.The petty small wars are stupid distractions by comparison.
edit: fixed awkward typo