In the summer of 1799, Muller's 18,000-man army had the mission of drawing Charles' Austrian army away from Switzerland, the central and western portions of which were held by André Masséna's army. Moving south from Mannheim, the "Army of the Rhine" laid siege to Philippsburg. Provoked by this threat to his strategic rear, Charles and 30,000 troops moved north against Muller, who quickly withdrew. Muller erred in leaving Laroche to be mauled by Charles, but his campaign became a strategic success when Masséna severely defeated a Russian army at the Second Battle of Zurich on 25 and 26 September.