Complex sentence: The Niedermayer-Hentig Expedition was a diplomatic mission sent by the Central Powers to Afghanistan in 1915-1916 that sought to encourage Afghanistan to declare full independence from the United Kingdom, enter World War I on the side of the Central Powers, and attack India.
Split into simpler sentences: The Niedermayer-Hentig Expedition was a diplomatic mission sent by the Central Powers to Afghanistan in 1915-1916. The purpose was to encourage Afghanistan to declare full independence from the United Kingdom, enter World War I on the side of the Central Powers, and attack India.