The EU and Turkey. Will they? Won't they? Could they just move in together for a few months and see how it goes?
The US has certainly given its blessing to the Union, though this has upset those who see parental blessing as outmoded and interfering.
Apart from the issues concerning Turkey itself (of which there are many [see further down]), the EU's "Turkish Question" is mostly about the EU itself, and how it sees itself (though everything seems to boil down to that in Europe nowadays). So here are 2 incomplete models (they overlap a lot, and they're also unsatisfactory in themselves anyway): Europe the geographical and Europe the cultural.
1. Europe the geographical:
Despite the fact that Europe is not really a continent in the geographic sense (it is a continent largely through a historical-cultural consciousness/tradition), there are geographically themed arguments for the limitation of Europe.
The EU should be limited to the European continent (and a limited version of that), since most of its benefits only make sense in that context: peace between European nations, strong and strengthening trade links with neighbours, plus cultural factors are heavily influenced by geography: history and common values can only spread so far and bind so many. By expanding the EU beyond Europe's "natural frontiers" (an idea heavily layered in political and cultural thought in itself), these common ties weaken - not just between older members and the newer, far-flung members, but between all members.
The common ties linked so heavily with geography cannot be so easily exported, especially since the EU model of integration leaves national identity and cultures largely intact: without a higher degree of assimilation, which would in itself detract from the European model, how could such an ever widening membership retain a common identity? Addition of new member states beyond a certain frontier is also of dubious value in pure materialistic terms: the greatest beneficiaries of integration are the core geographical countries: goods and services flow thickest between the original 6 and their neighbours due to geographical factors (though all countries benefit greatly from the single market). The further the member state is from the "core", the less the materialistic benefit to the older members. The opening of markets is a great benefit, but would geographical factors mitigate against the realisation of this potential, and render it a paper gain, whereas the costs remain real.
2. Europe the cultural:
This includes some of the ideas in the "geographical" model: a shared history and geography form the basis of common cultural ties. However, it moves beyond that to hold up some of the rationales underlying the EU (as opposed to the current form of the EU itself) as containing universalist ideals which are ripe for export to other countries, and that there can be a process of societal and value change which may result in a country sharing the same "top-level" of values: the welfare state, similar views on the relationship between the state and the individual, on the relationship between individuals and between states and peoples. This "top-level" of values and cultural modes of thinking would leave most local and national traditions and identities intact, but it would set them in a context suitable for membership of the EU without weakening the common values of the Union.
The cultural model places less store in the limiting factors of geography, and this applies to the material sphere: before the "big bang" expansion of 2004, there was no group of member states that could provide a counter-weight to the core. As the eastern European states grow more wealthy, they could form a "core" of their own - or more likely, give rise to a series of interlinking cores across the continent. Such a set-up would be more stable than the geographical model suggests, and all member states would benefit from a strong single market economy both economically, and politically in trade talks, etc. The "costs" to older member states would decline as the newer member states become more wealthy and develop equally strong trading links, and the benefits of their membership for the other members would remain and grow. The increasing power of technology as a communication tool and a trading tool also unlocks the potential of sustaining strong trade and economic links, as well as cultural links, across greater distances.
While the cultural model underplays the restrictions of geography, the restrictions of time are more acutely felt: values take longer to transmit than the acquis of the EU.
These are both terrible models - I'm leaving a lot out, and throwing in a lot of my own speculation, as well as implying a level of coherence and awareness about the models that doesn't translate into real life. But I've tried it to shed some light on what I think are a few of the arguments that underlie enlargement.
My own feelings for the moment lie somewhere in the middle, as a mix of both. Both are to some extent arguable with the models of ASEAN, Union for South American Nations, and the Africian Union. These organisations show that the universalist aspects of the EU model can be exportable - but perhaps that the limits imposed by geography, as well as political factors, mean that this will result in regional variants rather than EU expansion. (Granted, this is a crude comparison).
So where does Turkey fit in in all of this? It has some territory in what is classically considered Europe, and it has many of the same values that are considered shared European values. There are also big differences on both counts: the "European territory" is small, and Turkey faces legal reform challenges, especially on Human Rights.
Of course, there are also a lot of political reasons too: letting in a big, comparitively poor country raises issues of the balance of political power within the EU institutions and the strain on the EU budget, quite apart from any politically sensitive cultural differences. My personal preference is that Turkey should become a fully-fledged EU member - apart from all of the practical arguments, I feel that we have already made a committment to Turkey's eventual membership by opening negotiations, and it would be wrong to turn our backs on Turkey now - if anything else, it will reduce the EU's leverage in the region.
As for the big question of "Kleineuropa oder Grosseuropa?", I'm a Grosseuropa-ist at the moment, though I would put off accepting any more candidates for membership until the EU is institutionally ready for them.