In Italy, the Austrians under General Melas attacked first, and by the third week in April had advanced to the Var, with Massena and half his army in Genoa besieged by land, by the Austrians and under tight blockade by the Royal Navy. In response Berthier moved - not to the threatened frontier, but to Geneva - and Massena was instructed to hold Genoa until 4 June. The Army of the Reserve was joined by Napoleon, and in mid-May set out to cross the Alps to attack the Austrian rear. The bulk of the army crossed by the Great St Bernard Pass, still under snow, and by 24 May 40,000 troops were in the valley of the Po. Artillery was man-hauled over with great effort and ingenuity; however an Austrian-held fort on the Italian side (although bypassed by infantry and cavalry) prevented most of the artillery reaching the plains of Northern Italy until the start of June.