Russia’s invasion of Ukraine has revived EU expansion as an imperative. For years, “enlargement” has been a low priority – Croatia was the last country to join the club, more than 10 years ago. But things have changed. Ursula von der Leyen told a forum in Bled, Slovenia I attended this month that Europe’s security depends on the 27-nation union expanding again. Thanks to Vladimir Putin, as the Albanian prime minister, Edi Rama, put it, enlargement is back on track. The fact that expansion is a strategic priority makes it possible that new members will be admitted. But it doesn’t make it certain. It is about 20 years since six western Balkan countries (Albania, Bosnia and Herzegovina, Montenegro, North Macedonia, Serbia and Kosovo) were promised hope of membership, but little progress has been made since then. The Balkan countries don’t need the EU for security guarantees. Albania, Montenegro and North Macedonia are members of Nato, while Bosnia and Kosovo have a large Nato presence. It is a very different scenario from Europe’s east, where the absence of Nato’s shield to protect Ukraine, Moldova and the Caucasus means these countries view EU membership as a surrogate security guarantee. But Ukraine’s EU membership drive in particular has created a palpable fear in the western Balkans of being left behind. Serbia wants nothing to do with Nato, and its close relationship with Moscow has complicated Belgrade’s bid to enter the EU, even more so since Russia’s large-scale invasion of Ukraine. Yet, in important ways the latest momentum around EU enlargement is helping Balkan countries that aspire to join. Had it not been for the invasion of Ukraine, accession negotiations with Albania and North Macedonia would certainly still be stuck, and Bosnia and Herzegovina would not have been recognised as an EU candidate. Perhaps the EU would also not have agreed the budget for its new €6bn growth plan for the western Balkans. The plan makes European investments conditional on Balkan reforms, but if its full potential is realised, countries in the region could receive per capita almost as much money as full members are entitled to under the EU’s cohesion fund, which is designed to help poorer regions catch up. This would make economic convergence between the EU and the western Balkans more than just a dream for the first time. A number of projects to put flesh on the bones of the region’s gradual European integration are in the pipeline, such as these countries’ inclusion in the European single Euro payments area. While scepticism of the EU’s intentions still abounds in Albania, the conversations I had during a recent visit to Tirana went beyond the usual list of complaints about Brussels’ unfair treatment of the region. However, this is not enough. Von der Leyen’s next commissioner for enlargement should raise the level of ambition of what gradual integration can actually deliver, with carrots such as inclusion in the Schengen area considered. The negotiations over the EU’s next seven-year budget, which begin early next year and cover the 2028-2034 period, should also bake in the financial implications of including new members. Most important, steps must be taken to eject the main poison from existing enlargement policy. For too long, enlargement has been hijacked by the whims of existing members seeking to promote their national interests. Be this Cyprus and Greece over Turkey, Greece and Bulgaria over North Macedonia, France (in the past) and Greece over Albania, or Hungary over Ukraine, the exploitation of the accession policy by different members to further their (legitimate or otherwise) interests has dramatically hampered its credibility by constantly moving the goalposts for applicant countries. The main route through which this poison spreads is by giving EU governments an opportunity to block the process at every stage. Viktor Orbán’s Hungary doesn’t even try to hide this intention any more. Yet there is nothing in EU law that requires such interference by member states, which are only requested to greenlight the beginning and the end of the process. In previous enlargement rounds, the intermediary steps were correctly treated as technical matters in the hands of the Commission. It is possible politically and legally to revert to this method. What this requires is a first mover to trigger a critical mass. Germany and Slovenia have already proposed ways to unburden the process. Other member states supportive of Europe integrating the western Balkans, such as Austria, Croatia, the Czech Republic, Italy and Slovakia, should follow suit. Countries such as Poland and the Baltics, with an interest in Ukraine and Moldova’s EU membership, and recent converts to enlargement such as France, should all make their voices heard. There will be outliers such as Hungary, and perhaps Bulgaria, Greece and Cyprus. But as the history of European integration proves time and again, when a critical mass builds in favour of a decision, it is very difficult for recalcitrant members to resist for long. If supporters of an expanded EU are true believers, it is high time to show it through concrete policy action and not just words, less still questionable bilateral deals over migration such as that reached between Italy and Albania. Nathalie Tocci is a Guardian Europe columnist