 With America's entry into World War I, the battalion was expanded on 15 May 1917 to become the 1st Engineer Regiment, composed of six companies, and assigned to the 1st Infantry Division.As part of this expansion, Company C and Company D were reorganized to form the 6th Engineer Battalion the same day.A new Company C (the third company to bear the name), was formed to replace the company reorganized as part of the 6th Engineers.