Complex sentence: Throughout the rest of the world there are national markets established in various countries, this is dependent on the aviation activity within each country, the United States has a large percentage of the world's general aviation fleet and has a large established market.
Split into simpler sentences: Throughout the rest of the world there are national markets established in various countries, each dependent on the aviation activity within each country. The United States has a large percentage of the world's general aviation fleet and has a large established market.