In early 1778, Colonel George Rogers Clark offered Bowman a commission if he could raise his own company. Captain Bowman assembled his company and joined the rest of Clark's force on 12 May 1778, and was part of Clark's campaign. Bowman was present at the capture of Fort Gage in Kaskaskia, on 4 July 1778. Bowman later moved north, leading an non-uniformed, combined Virginian and local- Illinois Country, French/transplanted French Canadien- force, to capture Prairie du Rocher and Cahokia Fort de Chartres and Cahokia, where Bowman remained, for a time, as the commanding officer of the renamed Fort Bowman.