The First World War should have taught us how small events can lead to global cataclysms if ideologies are substituted for self-interest.

The seeds of the First World War — what an earlier generation called the Great War — were planted in the mid-to-late 19th century when Bismarck’s Prussia waged three wars to unify Germany.

It was announced after a meeting on Sunday between European Commission President Ursula von der Leyen and US President Donald ...