Complex sentence: Its proximity to New York City and favorable topography, made it especially good for air flights, and at the dawn of the aviation age, Long Island assumed a role as one of the, if not the, aviation capitals of the world.
Split into simpler sentences: Its proximity to New York City and favorable topography made it especially good for air flights. At the dawn of the aviation age, Long Island assumed a role as one of the, if not "the", aviation capitals of the world.