Complex sentence: Rural Attica was a primary land route for delivery of food sources such as livestock from Euboea to Athens, and this decimated Athens, which was concurrently being beaten in the Sicilian Expedition it had undertaken in the west.
Split into simpler sentences: Rural Attica was a primary land route for delivery of food sources such as livestock from Euboea to Athens. This was a serious blow to Athens, which was concurrently being beaten in the Sicilian Expedition it had undertaken in the west.