European Union

History

1951: The European Coal and Steel Community is formed, France, Italy, West Germany, Belgium, the Netherlands and Luxembourg are the first members.
1973: The UK, Ireland and Denmark (including Greenland) all join the EU.
1986: Spain and Portugal join the EU
1990: German reunification ensures East Germany is now a part of the EU.
1995: Sweden, Finland and Austria join the EU.
2004: Poland, Hungary, Cyprus, Czechia, Slovakia, Slovenia, Estonia, Latvia and Lithuania join the EU.
2007: Romania and Bulgaria join the EU.
2013: Croatia joins the EU.
2016: The Brexit referendum in the UK announces British intentions to the leave the EU.
2020: The UK formally leaves the EU.

The EU is the largest-scale example of open borders between nation-states and is largely successful.
No wars have broken out between EU member states since they have joined the EU. The continent that saw the two biggest wars in history soon became the most peaceful in the world.