 On May 1 the left wing of the Austrian army began crossing the Po River, forcing the French detachments on the southern bank to retreat toward Parma.Throughout May, that wing, commanded by Frederick of Württemberg, moved upriver toward Parma, while the right wing, under Marshal Mercy, moved more slowly, delayed by the need for provisions and a stroke suffered by the aging Mercy, that deprived him of his sight.On June 1, Frederick dislodged a detachment of French troops that were stationed to protect a bridgehead at Colorno, not far from Parma.