Complex sentence: The Louisiana Purchase of 1803 came during the peaceful lull after Napoleon decided against building a New World empire; he took Louisiana from Spain and sold it to the US for $15 million, including $11 million in gold.
Split into simpler sentences: The Louisiana Purchase of 1803 came during the peaceful lull after Napoleon decided against building a New World empire. So he took Louisiana from Spain and sold it to the U.S. for $15 million, including $11 million in gold.