Complex sentence: Second Empire is an architectural style, most popular in the last half of the nineteenth century and early years of the twentieth century, it was so named for the architectural elements in vogue during the era of the Second French Empire.
Split into simpler sentences: Second Empire is an architectural style, most popular in the last half of the nineteenth century and early years of the twentieth century. It was so named for the architectural elements in vogue during the era of the Second French Empire.