Enlargement of the European Union
In the first half of the 20th century, there had been two devastating wars that pitted the Germans against the French. A major motivation for the European Union, which began in 1957 as a free-trade area, was the hope that tighter economic and political integration among Europe's major powers would help prevent another war.
Over time, the EU has become both larger and more deeply integrated. Perhaps as a result, the continent's major powers have never fought another war against each other.