The cardinal mistake of our time was the decision to muddle through the eurozone crisis.
Europe’s political leadership failed to generate the public support for what was needed: creating a political and economic union.
With hindsight, the EU was wrong to construct a single currency without a proper banking union.
It was wrong to create a passport-free travel zone without a common border police force and immigration policy.
I would add EU enlargement to this list — not the principle but the haste with which it was pursued.