Complex sentence: The Channel Islands of California have been under the sovereignty of the United States since 1852, while some unofficial Mexican groups have demanded the country to take them back, but not the government of Mexico has not made any claim on them.
Split into simpler sentences: The Channel Islands of California have been under the sovereignty of the United States since 1852. While some unofficial Mexican groups have demanded the country take them back, the government of Mexico has not made any claim on them.