As mankind spread to the stars beyond, the U.S. government realized that there wasn't much uniting Americans in space beyond the fact that they called themselves Americans. They decided they need a national symbol, so they adopted a completely powerless monarchy to act as that national symbol. They remain general apolitical and avoid controversy; their lives are managed and scripted from birth, and are very much like reality TV stars. As the story begins, it's been about a century and the current monarch is Emperor Philip II.
His heir was his only child, Catherine, Princess of Columbia.
However, while the Imperial family was on a trip to the Caribbean, disaster struck. Debris entered the engine of the Imperial Eagle, the aircraft of the Imperial family, causing it to crash not long after takeoff. The crash killed Philip II and his wife Katherine; his daughter Princess Catherine, his younger brother Prince Andrew, Duke of Cascadia and his son and heir, Prince Edward.
Following the disaster, Prince Andrew's daughter, Princess Victoria, ascended to the throne as Empress Victoria. The sudden and unfortunate death of five members of the Imperial family threw the United States into an extended period of mourning. However, under the leadership of Empress Victoria and President Benjamin G. Thurman, the nation was able to recover. The White House and Observatory Palace had planned a lavish affair for Victoria's coronation.