Complex sentence: Underwood became Secretary of the Emigrant Aid and Homestead Society (which he incorporated with Massachusetts congressman Eli Thayer) from 1856-1861, and worked to encourage emigration of Republicans and European emigrants to the Ohio Valley counties of Virginia (that would become West Virginia).
Split into simpler sentences: Underwood became Secretary of the Emigrant Aid and Homestead Society (which he incorporated with Massachusetts congressman Eli Thayer) from 1856-1861. He worked to encourage emigration of Republicans and European emigrants to the Ohio Valley counties of Virginia (that would become West Virginia).