Let us now look at the normative foundations of an enlargement of the EU. The legal basis for an accession is Art. 49 E.U. Treaty, which regulates the admission procedure.29 The conditions of admission, and the necessary adaptations of those treaties which form the basis of the Union, are fixed by an agreement between the Member States and the applying state.30 This agreement needs to be ratified by all Member States in accordance with their constitutional provisions. Since the Maastricht Treaty, accession occurs as accession to the EU and, at the same time, to the three European Communities.31 An accession, therefore, refers to the whole institutional frame of the EU, including the Communities. However, isolated accessions were not practiced before either.32 Despite the uniform procedure, a newly admitted state is still acceding to the individual Communities and, if necessary, the individual Community treaties need to be adapted.33
As to the substantive conditions for accessions, Art. 49 E.U. Treaty only contains very few. Apart from the fact that the accessing state needs to be a European one,34 it explicitly only demands the adherence to certain fundamental legal principles which, according to Art. 6(1) E.U. Treaty, count at the same time as the foundation of the EU. These are the common structural principles and fundamental rights of the Member States, namely the principles of freedom, democracy, respect for human dignity and the basic freedoms as well as the rule of law.35
The Treaties do not define the extent to which the new Member States need to be integrated into the EU and the Communities. As seen, the Community bodies in this respect always, since the first accession, have looked to preserve the complete acquis communautaire for the whole Community even after the accession of new Member States.36 A fulfillment of this requirement is to be expected, because the candidates for accession do not possess a legal title to accession,37 at least not in a general form, rooted in Art. 49 E.U. Treaty or another rule of primary Community law.38 This renders it possible for the EU to set the conditions for an accession politically and does not say anything about the legal limits. We only need to note for the moment that Art. 49 E.U. Treaty does not require a total adoption of the Community and Union law or the acquis communautaire.39 It also does not contain rules defining the extent of admissible exemptions from Community law or defining the conditions under which such exemptions are admissible at all.40 More exact guidelines in this respect can only be deduced from general legal principles.
Without going into the question - depending on the point of view either difficult or simple - of substantive limits for a modification of the Treaty generally,41 my assumption is that, at least in the case of the enlargement of the EU, the fundamental principles of the EU and the Communities need to be preserved. This is because basically the procedure for admitting new Member States is designed to maintain the identity of the admitting institution.42 This is true even if an accession, and especially the enlargement by several new Member States, can be - and possibly must be - at the same time taken as an opportunity for reforms.
The differentiation between enlargement and treaty modification is clearly expressed in the E.U. Treaty. Modifications of the treaties necessitated by an accession can be made during the course of the accession procedure according to the already mentioned Art. 49(2) E.U. Treaty. This means primarily changes in the composition of the Community bodies, but also adoption of those transition regulations which are thought necessary.43 If, however, changes to the Treaties are to be made solely on the occasion of an accession, and if those changes affect the existing system of Community law, a Treaty modification on the basis of Art. 48 E.U. Treaty is needed.44 Admittedly, the differentiation between an adaptation and other (more substantive) changes is not always easy and also not undisputed.45 Furthermore, modifications might be taken within an accession procedure, as this procedure is more complicated than that of the mere Treaty modification. However, the above-mentioned differentiation is rooted in the Treaty itself.
Therefore, enlargements of the EU that are not simultaneously aimed at reforming Community law need to leave fundamental principles of the Community unharmed. This means all principles which define the identity of the Community.46 Part of these is, surely, the institutional structure of the Community.47 Among the fundamentals are the principles mentioned in Art. 6 E.U. Treaty48 and fundamental rights, supranationality of the EC,49 and also the principle of integration as well as the principles expressed in the policies, above all free market and competition.50 The most important part of the last principle are the basic freedoms in the internal market. Their meaning was emphasized again and again by the ECJ during the course of its jurisdiction. Art. 39 et seq. E.C. Treaty states: "The Court has consistently held that the free movement of workers is part of the fundamental principles of the Community."51 One can conclude that an abolition or fundamental reduction of individual freedoms affects the identity of the Community and, therefore, is ruled out in the context of accessions as a matter of principle.
Before taking a closer look at the principles which have to be considered at accession, we first have to deal with an argument that refers to the relationship between transition regulations and the level of integration at the time of the accession. One could imagine that, in principle, transition measures are thought inadmissible because of the aims of integration already achieved. In concreto, the argument could be that, after creation of the internal market, exceptions from the basic freedoms, especially from the free movement of workers, are prohibited as far as they reach beyond those contained in the E.C. Treaty itself.
This, however, cannot be reasoned in a convincing manner.52 Apart from the fact that the practice at earlier enlargements did not stay within certain limits, transitional measures serve the approximation to a certain level of integration. Thus it is not relevant if an integration level satisfying at least the current objectives of the Treaties is already achieved. Indeed, those objectives can change. Also, the objective of creating an internal market53 is not endangered by approximations on the part of new applying states. It is possible that, after abolition of all border controls, the limited re-introduction of these controls can be regarded as inadmissible because of the related strong infringements. Another exception to the right of free movement of workers would, however, remain possible.
29 It reads: "Any European state which respects the principles set out in Article 6(1) may apply to become a member of the Union. It shall address its application to the Council, which shall act unanimously after consulting the Commission and after receiving the assent of the European Parliament, which shall act by an absolute majority of its component members. The conditions of admission and the adjustments to the Treaties on which the Union is founded which such admission entails shall be the subject of an agreement between the Member States and the applicant State. This agreement shall be submitted for ratification by all the contracting States in accordance with their respective constitutional requirements."
30 For the opinions of the Commission which are of importance for both steps see Booß/Forman, Enlargement: Legal and Procedural Aspects,  CMLR, pp. 95, 98. As to the roles played by different institutions in the accession process, see Weatherill/Beaumont, EC Law, 2nd ed. (1995), p. 130 et seq.
31 See Art. 1(3)1 E.U. Treaty: "The Union shall be founded on the European Communities, supplemented by the policies and forms of cooperation established by this Treaty."
32 As to the expansions see above II 1. After the fusion of the bodies, coming into force on July 1st, 1967, isolated accessions would hardly be possible. Also see Much, Rechtliche Grundsatzfragen zur Erweiterung der Europäischen Gemeinschaften, EuR 1972, p. 324, 329.
33 Whereby one can debate if the accession to the Communities happens as accession via the EU as single organization (so v. Bogdandy/Nettesheim, Die Verschmelzung der Europäischen Gemeinschaften in der Europäischen Union, NJW 1995, pp. 2324, 2326) or if Art. 49 E.U. Treaty only combines the regulations as to the accessions to the Community and to the Union (so Koenig/Pechstein, Die Europäische Union, 1st ed. (1995), no. 8). For the differences in structure of the different Community treaties see Kapteyn/VerLoren van Themaat, Introduction to the Law of the European Communities, 3rd ed. (1998), p. 68 et seq.
34 This condition was the decisive factor for the rejection of Morocco's application for accession in 1987. It does not hinder Turkey's accession.
35 See Meng, in: Groeben/Thiesing/Ehlermann, EU-/EGV, 5th ed. (1997), Art. O EUV, no. 52 ff.; about the rule of homogeneity, see Nicolaysen, Europarecht I, S. 78. More detailed to the (at this time still unwritten) conditions of accession and the relevant jurisdiction of the ECJ see Ehlermann, Mitgliedschaft in der Europäischen Gemeinschaft - Rechtsprobleme der Erweiterung, der Mitgliedschaft und der Verkleinerung, EuR 1984, p. 113, 114 et seq. As to the unwritten conditions of (the former) art. O E.U. Treaty also see Richter, Die Erweiterung der Europäischen Union (1997), p. 28 et seq.
36 Compare above, III.2.
37 See Oppermann, Europarecht, 2nd ed. (1999), no. 1842; for the same situation in public international law see Seidl-Hohenveldern, Völkerrecht, 9th ed. (1997), no. 318.
38 Whether the EC can contractually bind itself towards the candidates for accession is another question. The beginning of accession negotiations or integrating measures alone do not yet create any obligation, and the preparation for the accession by the accession candidates does not cause any legitimate expectations.
39 More far reaching Oppermann, Europarecht, 2nd ed. (1999), no. 1853, who equates the adoption of Community law with the upholding of the Community order. Nicolaysen, Europarecht I, p. 78 takes it for a condition for accession, however without naming a legal basis. Merely descriptive Booß/Forman (footnote 30),  CMLR, p. 95, 100 et seq.
40 Meier, Die rechtlichen Grenzen für einen Beitritt zu den Europäischen Gemeinschaften, EuR 1978, pp. 12, 16 et seq., 20, deducts from the requirement of "accession conditions", mentioned in the E.C. Treaty (and earlier in the E.E.C. Treaty), that the Member States are obliged by Community law to ensure at the accession "that the Community's level of integration or certain aims of integration or the Community's ability to action is not seriously endangered by the accession". This should lead, in the end, to a prohibition of accessions for states without a sufficient economic level, p. 24.
41 Which as such naturally can be relevant for all treaty changes and therefore also for those related to an accession, see for the different models of reasoning Becker, EU-Erweiterung und differenzierte Integration (footnote 19), p. 64 et seq., with further references.
42 See Oppermann, Europarecht, 2nd ed. (1999), no. 1855.
43 See Beutler, in: Beutler/Bieber/Pipkorn/Streil, Die Europäische Union, 4th ed. (1993), p. 48; Vedder, in: Grabitz/Hilf, EUV (1999), Art. 237 EWGV, no. 24.
44 The formal requirements of art. 48 E.U. Treaty, and also the question if they are disposable, do not need to be explained here; hereto, see Koenig/Pechstein, Die EU-Vertragsänderung, EuR 1998, p. 130 et seq.
45 See Meng, in: Groeben/Thiesing/Ehlermann, EU-/EGV, 5th ed. (1997), Art. O EUV, no. 125 ff.; especially one can assume that all changes made because of an accession fall under art. 49 E.U. Treaty as lex specialis. As to the attempts of the Commission to reach, by regulating the accessions, modifications of Community law which were to avoid a feared clumsiness, see Ehlermann, Mitgliedschaft in der Europäischen Gemeinschaft - Rechtsprobleme der Erweiterung, der Mitgliedschaft und der Verkleinerung, EuR 1984, p. 113, 116 et seq.
46 Insofar as a core of non-changeable rules and therefore a substantive limit for all treaty changes (also those not related to an accession) is assumed, opinions differ as to if this core is identical with those principles defining the Community's identity, see Meng, in: Groeben/Thiesing/Ehlermann, EU-/EGV, 5th ed. (1997), Art. N EUV, no. 54 et seq. who sees the principle of integration and the economic system as non-changeable.
47 Without however excluding certain modifications of competences and functions.
48 In detail, see Kapteyn/VerLoren van Themaat, Introduction to the Law of the European Communities, 3rd ed. (1998), p. 135 et seq.
49 Ipsen, Europäisches Gemeinschaftsrecht (1972), 2/44 et seq.
50 For a relatively wide definition of constitutional principles see Zuleeg, in: Groeben/ Thiesing/Ehlermann, EU-/EGV, 5th ed. (1997), Art. 1, no. 43 et seq.
51 Case 139/85, Kempf  ECR 7141/13.
52 See Becker, EU-Erweiterung und differenzierte Integration (footnote 19), p. 71 et seq.
53 The differences between common market and internal market are only of gradual character. Especially the program of the internal market serves to speed up the rather slow process of integration, see Schwarze/Becker/Pollak, Die Implementation von Gemeinschaftsrecht (1993), p. 15 et seq.; also see Steindorff, Gemeinsamer Markt als Binnenmarkt, ZHR 150 (1986), S. 689. The exact relationship between common and internal market is discussed controversially in the literature; no new elements expects Glaesner, Die Einheitliche Europäische Akte - Versuch einer Wertung, in: Schwarze (ed.), Der Gemeinsame Markt - Bestand und Zukunft in wirtschaftsrechtlicher Perspektive (1987), p. 16 et seq. For the opinion that the internal market is not only a deeper but also a more comprehensive level see Ehlermann, The Internal Market Following the Single European Act,  CMLR, p. 364 et seq.; more restricted Forwood/Clough, The Single European Act and Free Movement,  ELR, p. 385.