Complex sentence: To be permitted entry to the United States, thousands of mostly Chinese immigrants crossing the Pacific to San Francisco had to enter through the gauntlet of Angel Island, and were detained for months in a purgatory of isolation.
Split into simpler sentences: To be permitted entry to the United States, Chinese immigrants crossing the Pacific to San Francisco had to enter through the gauntlet of Angel Island. Thousands, the majority being Chinese, were detained for months in a purgatory of isolation.