Complex sentence: The civil war left Greece with a legacy of political polarization; as a result, Greece also entered into alliance with the United States and joined NATO, while relationships with its Communist northern neighbours, both pro-Soviet and neutral, became strained.
Split into simpler sentences: The civil war left Greece with a legacy of political polarization. As a result, Greece also entered into an alliance with the United States and joined NATO, while relationships with its Communist northern neighbours, both pro-Soviet and neutral, became strained.