Complex sentence: While he was a student at Harvard, Marquand joined Battery A of the Massachusetts National Guard, in 1916 this unit was activated and in July 1916 Marquand was sent to the Mexican border.
Split into simpler sentences: While he was a student at Harvard, Marquand joined Battery A of the Massachusetts National Guard, which, in 1916, was activated. In July 1916, Marquand was sent to the Mexican border.