Beyond EU principles supported by national governments and Barnier's inclusive method to build a common front, the unity was rooted in a sense of political responsibility of prime ministers for the survival of the European integration project. The eurozone was just healing from raucous divisions. In 2015, the Greek people rejected in a referendum an EU aid package with strings attached. The vote pushed the country to the cliff-edge of leaving the eurozone. US President Obama, Juncker, Hollande, Merkel and ultimately the Greek Prime Minister Tsipras pulled the country back from the cliff. Merkel spent political capital against conservative politicians in her own party who wanted Greece out. Two months later, the migration crisis and Merkel's “wir-schaffen-das”-welcome to Syrian refugees travelling through Hungary put enormous pressure on the Schengen border-free travel zone. New terrorist threats emerged. In March 2016, terrorists bombed Brussels airport and the Maalbeek metro station close to Schuman, killing more than 30 people, a few months after the gruesome Bataclan and Stade de France attacks in Paris. Voices in Russia, which invaded Crimea two years earlier, welcomed Brexit as a weakening of the EU and a possible lifting of sanctions now that a “hardline UK” was leaving. This was a time for the EU to stand together faced with the possibly existential threat of the UK's departure. EU presidents affirmed on 24 June 2016: “the Union of 27 member states will continue” and leaders decided to meet soon in Bratislava to discuss how to move European integration forward. As often in difficult times, a roadmap for future and deeper cooperation offered an element of comfort to EU leaders.

In the months after the June 2016 referendum, some governments feared Brexit might trigger a domino effect on their country. Rising populism and Euroscepticism led to speculation on Nexit, Frexit, Swexit and other neologisms that popped up in tandem with the popularity of the Dutch Geert Wilders, the French Marine Le Pen and the anti-European Swedish Democrats of Jimmie Äkesson. Mark Rutte of the Netherlands had in mind the March 2017 elections, and people in Brussels looked with concern at Le Pen's polling scores before the first round of the presidential elections one month later.