The European Union?
I was created long ago, more than fifty years in fact.
Back then there were just six in me, held together by a pact.
Created in 1957 between Belgium, France, Italy, Luxembourg, the Netherlands and West Germany, via the Treaty of Rome
Next I did increase by three, but one of them will leave.
Then one more came in alone, and I did almost grieve.
Denmark, Ireland, and the U.K joined, the U.K will leave. (Brexit) Greece then joined, and is the EU economy currently struggling most. (grieving?)
My next addition was a pair, but they were both close by.
Then did come another three, and I extended high.
Portugal and Spain joined next, Germany was reunified(not mentioned) and then Austria, Sweden, and Finland joined. This extended the EU to the north (high)
One summer warm some years ago, I grew and grew in size.
The largest change I'd ever seen - so many new allies.
To quote Wikipedia "In 2004, the EU saw its biggest enlargement to date when Cyprus, the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, Slovakia and Slovenia joined the Union."
Two more and then a final one, my history is complete.
Identify me and you'll get - a checkmark for a treat.
Romania and Bulgaria, followed by Croatia