Complex sentence: There are various factors that make it difficult to determine when exactly Hispanics began to serve in the Corps; one is that statistics on Hispanics were not kept by the military until the 1970s when the United States Census Bureau coined the phrase.
Split into simpler sentences: There are various factors which make it difficult to determine when exactly Hispanics began to serve in the Corps. One of these factors is that statistics on Hispanics were not kept by the military until the 1970s when the United States Census Bureau coined the phrase.