If the city-states of Ancient Greece had united into one entity, the History of the world 450 BC - present would be different
If the states of America were not united & thus had not joined WWI and WWII, the history of the world 1914-present would have been different.
If the countries of Europe do not unite properly, the history of the world 2012- ..... will be different than if they do.
PS: Food for thought:
At different times, Athens, Sparta, Thebes & Macedonia failed to create a United States of Greece. They abused their power over the others. These days, the "Sparta" & "Athens" of Europe, Germany & France, are trying something, together! But are their leaders doing it properly or alienating the other member states (by analogy to the Ancient Greek city-states).
Is Greece the "Delphi" of modern Europe? Is France the "Athens"? Is Germany the "Sparta"? What is the UK? Spain? Finland?