EU leaders have marked the 60th anniversary of the bloc's founding treaty as a historical turning point, with the UK preparing to officially trigger divorce proceedings next week.
In their solemn declaration ending the summit, the 27 leaders sought to end paralysis which has gripped the EU on several issues and enshrined the notion that some member states will be able to move ahead on some issues while others remain on the sidelines.
European Commission president Jean-Claude Juncker called Brexit "a tragedy", while EU Council president Donald Tusk said sustained unity is the only way for the EU to survive.
Speaking to EU leaders at a solemn session in precisely the same ornate hall on the ancient Capitoline Hill where the Treaty of Rome founding the EU was signed on March 25 1957, Mr Tusk said: "Europe as a political entity will either be united, or will not be at all.
"Only a united Europe can be a sovereign Europe in relation to the rest of the world.
"Only a sovereign Europe guarantees independence for its nations, guarantees freedom for its citizens."
However, in order to move forward, the leaders recognised that full unity on all things will be unworkable.
The declaration signed by the 27 nations said: "We will act together, at different paces and intensity where necessary, while moving in the same direction."
The EU has often done that in practice in the past, with only 19 nations in the eurozone and not all members participating in the Schengen zone of borderless travel.
In a series of speeches, EU leaders also acknowledged how the bloc had strayed into a complicated structure which has slowly lost touch with its citizens, compounded by the financial crisis which struck several member nations over the past decade.
Italian prime minister Paolo Gentiloni, who was hosting the summit, said that over the past dozen years the EU's development had stalled.
"Unfortunately, we stopped," he said, "and it triggered a crisis of rejection."
However, there was also a message of optimism at the meeting in the bright spring sunshine.
Mr Gentiloni said: "Yes, we have problems, yes there are difficulties, yes there will be crisis in the future - but we stand together and we move forward.
"We have the strength to start out again."
At the end of the session, all 27 leaders signed a new Rome Declaration, stating that "European unity is a bold, far-sighted endeavour".
It added: "We have united for the better. Europe is our common future."