Complex sentence: On 1 July 1846 Captain James Allen, dispatched by Colonel (later Brigadier General) Stephen Kearny, arrived in the Mormons' Mosquito Creek camp with a request from President Polk to enlist a battalion of 500 volunteers to fight in the Mexican War.
Split into simpler sentences: On 1 July 1846 Captain James Allen, dispatched by Colonel (later Brigadier General) Stephen Kearny, arrived at the Mormons' Mosquito Creek camp. He had a request from President Polk to enlist a battalion of 500 volunteers to fight in the Mexican War.