As the European Union re-centers enlargement on its political agenda, the legal mechanics of treaty change face renewed scrutiny. While the accession of new Member States is legally grounded in Article 49 TEU, the distinction between “adjustments” and “amendments” remains both underexplored and constitutionally significant. With geopolitical shifts accelerating enlargement discussions, questions emerge about whether institutional reform should precede or accompany expansion. Can the EU truly deepen and widen simultaneously — or must treaty revision come first? This article explores the legal logic and limits of treaty adjustment, arguing for a clearer separation between constitutional amendment and technical accession adaptation.

Throughout most recent history, enlargement was a second-order priority for the European Union (hereinafter EU), never off the radar but never really a top agenda item either. However, recent geopolitical developments, especially the conflict between Ukraine and Russia, have had a significant impact on the EU’s enlargement policy and shed light on how the EU balances its internal values and principles with external factors, as well as how the EU would change as a whole, when increasing the number of Member States.

Reality poses a challenge to, as some argue, one of the most successful and impressive political transformations of the twentieth century that contributes to sustainable stability and security. Enlargement became the priority again.

For each country to become a member of the EU, it must fulfil a series of political, economic, and legal requirements, collectively referred to as the Copenhagen criteria, established in 1993. The process of a state’s accession is multifaceted, beginning at candidacy status, membership negotiations, and ending in accession. In the latter, the accession treaty, which details the terms and conditions of the country’s EU membership is signed and ratified by all EU Member States and the candidate country. The legal basis for the procedure of accession is Article 49 of the Treaty of European Union (hereinafter TEU). Beyond this, the Treaties do not explicitly outline specific objectives for the enlargement policy. The text of Article 49 TEU goes as follows:

“Any European State which respects the values referred to in Article 2 and is committed to promoting them may apply to become a member of the Union. The European Parliament and national Parliaments shall be notified of this application. The applicant State shall address its application to the Council, which shall act unanimously after consulting the Commission and after receiving the consent of the European Parliament, which shall act by a majority of its component members.

The conditions of eligibility agreed upon by the European Council shall be taken into account. 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.”

Article 49(2) provides the general basis for a state’s accession to the EU. The accession treaty of the new Member State may make “adjustments” to the Treaties. What do these adjustments mean? We can see that it acknowledges the possibility that an accession treaty may require modifications to the existing Treaty. It follows from the wording, which provides that adjustments are to be the subject of the accession treaty, meaning they are the necessary consequence of the accession of a new Member State. At first glance, it may seem that these are the very same procedures as in the previous Article 48 TEU, which talks about the “amendments” of the Treaties. Of course, there are some similarities, as they both require unanimity among states and need ratification under national constitutional requirements.

However, it is important to note that amending the Treaties is a complex constitutional matter. It changes the powers of the institutions or moves the competencies conferred on the Union. Article 48 is a legal basis for Treaties to be amended by an ordinary revision procedure, written in provisions two to five. Another possibility of amending lies in provisions six and seven, a foundation for changing the Treaties with simplified revision procedures as an exception to the ordinary procedure. As the EU went through enlargement, it also did the deepening. Various amendments to Treaties took place that would strengthen the political requirements the Copenhagen criteria had imposed. For example, The Treaty of Amsterdam, in Article 49, for the first time recognized the principles of liberty, democracy, respect for human rights, fundamental freedoms, and the rule of law, as conditions for EU membership. In an early judgment, the Court indicated that EU Treaties might only be modified under the procedure explicitly aimed at this, i.e. through the amendment procedure.

Meanwhile, the adjustment is a mere correction, after the candidate country meets the required measures for the accession. Accession Treaties were agreed unanimously by Member States and then ratified by national parliaments. Already seen at the 1969 Hague Conference of the Heads of State or Government, the six Member States decided to enlarge the Community in the following terms: “In so far as the applicant states accept the Treaties and their political aims, the decisions taken since the entry into force of the Treaties and the options adopted in the sphere of development […] indicated their agreement to the opening of negotiations between the Community on the one hand and the applicant States on the other.” Therefore, the candidate is responsible for adapting its legal order to make it well-suited to that of the EU, which was reaffirmed during later enlargements. We can describe the enlargement rules as conservative in view of their dogmatic emphasis on the candidate’s acceptance of the acquis, as well as non-interventionist, in that the EU leaves it almost entirely to the candidate to prepare itself for accession and fulfil the entry conditions.

Some argue, and we think rightfully so, that in practice, accession negotiations would have been endless if the parties were in the position to use the opportunity to renegotiate big portions of the Treaties. After all, the candidate is acceding to the EU and not vice-versa. In a recent judgment, the Court held that a candidate state seeking EU membership must ensure its constitution aligns with the fundamental values upon which members built the Union. On the other hand, it is important to note that this does not come without exceptions. The accession of Britain, Ireland, and Denmark to the EEC in 1973 marked the introduction of a new chapter on the common fisheries policy into the Treaty of Rome. That was because of the common fishing industries of both the incoming countries and Member States. However, a case could be made here that, back then, the community was mostly economic. As of now, especially after the Treaty of Lisbon, the character of the EU has become more political and constitutional. Therefore, the suitability of that amendment is at least questionable.

Yet, it begs the question of whether processes of amending and adjusting are not in some way connected and maybe we can freely choose between the two. Do amendments enshrined in Article 48 and adjustments in Article 49 overlap? Arguably, there is no real overlap between the provisions, meaning there is no free choice between them. The use of “shall” in Article 49, as opposed to “may” in Article 48(6) and (7), indicates that Article 49 serves as a lex specialis in relation to Article 48. Additionally, the word “shall” implies that certain adjustments to the Treaties are mandatory when drafting accession treaties. Any Treaty revision procedure other than the ordinary Treaty revision procedure is a derogation from the normal rule. Article 49 TEU covers adjustments to the Treaties, which are connected to the acceding states, as opposed to the withdrawal treaty under Article 50 TEU, which does not function as an additional Treaty revision procedure. Therefore, it constitutes a form of Treaty revision, but as an exception rather than the rule. The process of negotiating an accession treaty is different from the ordinary revision procedure. There is no Convention, IGC process, or formal role for the European Council except the defining eligibility conditions. Article 49 TEU grants the European Parliament a right to consent, unlike the ordinary revision procedure except regarding the choice between Convention and immediate calling of an intergovernmental conference; nor is it the case for the simplified revision procedure of Art. 48(6) TEU. Therefore, using Article 49 TEU to amend the Treaties would be an improper application of the procedure. Nevertheless, the difference between the two terms is obvious if we examine the CJEU’s case law.

In the following case (C-420/07, Apostolides) the Court made it clear that adjustments should be interpreted narrowly and restrictively: “Provisions in an Act of Accession which permit exceptions to or derogations from rules laid down by the EC Treaty must be interpreted restrictively with reference to the Treaty provisions in question and must be limited to what is absolutely necessary in order to attain its objective.” We see in the case of derogations that any adjustments that originate from any acts of accession are not to be applied broadly or leniently, but rather that the scope must remain limited and tightly aligned with the purpose of the relevant Treaty provisions and their objectives.

In regards to the adjustments in the acts of accession, in C‑273/04 (Poland v. EU and Commission), the Court, calling them “adaptations”, ruled that “in that regard, the Court has already ruled on the meaning of ‘necessary adaptations’ in the context of acts of accession” and further ruled that “adaptation measures provided for by such acts, as a general rule, authorize only adaptations intended to render earlier Community measures applicable in the new Member States, to the exclusion of all other amendments.” The decision indicates the non-inclusiveness of any amendments when the adjustments are added to the Treaties. Member States do not renegotiate the acquis together with the candidate state but instead address technical modifications, such as the allocation of the new Member State’s seats in the European Parliament and other committees, along with its voting rights in the Council. Aside from these numerical adjustments and transitional periods, the negotiation process primarily functions as a take-it-or-leave-it principle.

Last but not least, the term “adjustments which such admission entails”, as in Article 49 TEU, signifies an inherent causal connection between accession and the necessary amendments to the text of the Treaties. This clarifies that the permissible adjustments are solely those that arise naturally from accession, such as the expansion in the number of members of an institution to guarantee representation for the new countries. Enlargement treaties may only incorporate Treaty amendments that are strictly necessary for accommodating new Member States. These include references to the specific Member State joining, adjustments reflecting the updated number of Member States, as increases in the number of members of the European Parliament, and amendments related to derogations from Treaty rules, which constitute the “conditions of admission.” From the above, we can conclude that the term adjustments from 49 TEU is more limited than the amendments from 48 TEU. In our view, Article 49 serves a specific purpose — facilitating accession — whereas Article 48 is reserved for institutional and constitutional revisions. They cannot operate interchangeably and avoidance of the use of Article 48 to change the treaties would be a backward step for the polity’s emerging democracy. It is a matter of when, and with what purpose, the modification of Treaties takes place. Only then, we can talk about deepening and widening by the rules.

Nevertheless, nothing is stopping political connections from being drawn between accession treaties and broader Treaty revisions. This could mean postponing accession negotiations until wider Treaty amendments have been negotiated or ratified. The other option would be agreeing to broader amendments that address challenges arising from past enlargements. This only means that even if the adjustments and amendments are two different types of modification of Treaties, nothing prevents the Member States, the masters of the Treaties, from customizing the deepening and widening of the Union as they see fit. If a candidate state were allowed to amend the Treaties upon accession, it would imply that a non-member is participating in the constitutional reconfiguration of the Union it seeks to join — a notion that appears fundamentally paradoxical. In our view, adjusting should not be confused with amending the Treaties of Member States, which is an act of agreement between States already upholding the values of the Union.

Lovro Ušeničnik has a BA in Security Studies at the Faculty of Criminal Justice and Security (University of Maribor) and is now pursuing his MA in Law at the European Faculty of Law (New University). He served as a volunteer researcher at Transparency International Slovenia and participated in election observation missions both domestically and internationally. His research interests include security studies, and European and international law.