Complex sentence: Leonard broke in with the Boston Red Sox in 1913 and rose to fame in 1914 when he led the American League in ERA with 0.96, the modern record for single-season ERA.
Split into simpler sentences: Leonard broke in with the Boston Red Sox in 1913. In his second year in the major leagues, 1914, Leonard led the American League with a remarkable 0.96 ERA--still the record for single-season ERA.