Complex sentence: By 2008, "Halo 3" had sold 4.8 million units in the United States for a total of 8.1 million units worldwide, making it the best-selling game of 2007 in the United States.
Split into simpler sentences: By 2008, "Halo 3" had sold 4.8 million units in the United States for a total of 8.1 million units worldwide. The game was the best-selling game of 2007 in the United States.