Ilham Aliyev of Azerbaijan pulled out of the gathering in Granada, Spain, at the last minute, robbing the summit of one of its key deliverables, an EU-brokered meeting with his Armenian counterpart to diffuse tensions in the Caucasus.
Turkish President Recep Tayyip Erdoğan, a key ally of Aliyev, pulled out citing a cold.
The European Political Community (EPC) summit on Thursday will now see 45 leaders from across Europe gather to discuss common challenges and talk shop in an informal format. They will first gather in four working groups to discuss multilateralism, the environment, and digital issues including artificial intelligence with the war in Ukraine as the overarching theme.
They will then have a few hours to talk during bilateral meetings, which is when European Council President Charles Michel was hoping to sit down with Aliyev, Armenian Prime Minister Nikol Pashinyan, French President Emmanuel Macron and German Chancellor Olaf Scholz in a repeat of the previous two EPC summits.
"It is a shame that Azerbaijan is not here and that Turkey, the main country that supports Azerbaijan, is not here," Josep Borrell, the EU's top diplomat, told reporters, deploring the fact a meeting will not be held.
"I hope that there can be meetings in Brussels to prevent the conflict from escalating," he added.
The three EU leaders will however sit down with Pashinyan together.
'Great responsibility on the side of Azerbaijan'
Aliyev's decision not to attend the summit was down to an "anti-Azerbaijani atmosphere", an Azerbaijani official told AFP.
The summit comes just two weeks after the country's forces launched a lightning-quick military offensive to regain control of Nagorno-Karabakh from Armenian separatists, prompting a mass exodus of ethnic Armenians from the enclave and fuelling fears of ethnic cleansing.
Russian peacekeepers stationed in the mountainous region since a 2020 Russia-brokered peace deal that followed a brief but deadly conflict stood aside.
The EU condemned Azerbaijan's offensive with Michel telling Euronews earlier this week that he was "extremely disappointed" by the Azeri decision. He called for negotiations to "pin down commitments from both sides," stressing, however, that "there is a great responsibility on the side of Azerbaijan, which launched this military operation."
The EU has also pledged millions in humanitarian aid to Armenia while France, on Tuesday, announced it would deliver military equipment to Yerevan
Tensions in the Balkans should also feature high on the agenda with a meeting between Serbian President Aleksandar Vučić and Vjosa Osmani, President of Kosovo, also a strong possibility. However, as Osmani's role is largely ceremonial, a breakthrough is unlikely.
Kosovo has accused Serbia of being behind the September 24 attack in which one policeman was killed in northern Kosovo. Belgrade has denied any involvement.
Osmani, however, told reporters that she would ask "for the support of European allies and partners to sanction Serbia so that they are not encouraged and emboldened to commit such acts of aggression against Kosovo and against other neighbours again and again."
'A productive day for Ukraine and Europe'
A leader in high demand at the EPC summit is Volodymyr Zelenskyy.
"First of all, the main challenge for all of us that we have is to save unity in Europe," he told reporters upon arrival, warning that Russia will aim to splinter that unity with "attacks by disinformation, by fakes"
He also said that it is "so important for Ukraine to have defending shields for the winter because in the winter we will have many attacks by different kinds of Russian missiles, including Iranian drones".
Ahead of his arrival, he had said he was bringing "substantial proposals" to enhance the European security architecture, adding that he would work to finalise "new agreements" to strengthen air defence.
"This should be a productive day for Ukraine and Europe as a whole," he added.
Zelenskyy held bilaterals with the Spanish and Italian leaders as well as Macron.
Migration also made it on the agenda with Italian Prime Minister Giorgia Meloni and her British counterpart, Rishi Sunak, to host a "side event" in the afternoon on the issue.
A diplomatic source said Mark Rutte of the Netherlands and Edi Rama from Albania -- two countries on the frontline and two countries of destination -- were among those attending the gathering that will focus on possible operational initiatives to combat human trafficking.
The EPC summit will be followed on Friday by an informal gathering of the European Council, where the 27 leaders of the EU are expected to discuss enlargement, migration and the so-called Strategic Agenda, which sets out priority areas for the bloc for the next legislative term.