How did the geography of Greece impact Greek History?

The largest geographical impact on Greece was the fact that much of it is coastal.  Most of Greece is a peninsula.  Greece is surrounded by the Mediterranean Sea, the Ionian Sea, and the Aegean Sea.  Off the mainland are thousands of islands.  

Being located on the water had both positive and negative impacts on Greece.  Trade was important in ancient Greece.  Major cities, such as Athens, were located on or near the three seas.  Traders from Asia, Africa, and other parts of Europe could easy access Greece by water.  Greek traders could easily access other places in the world by sea.  This had a positive impact on the ancient Greek economy.  Major cities on the water were vulnerable to attacks by Greece's enemies.  This was the negative impact of the majority of Greece being located on the coast.