Outlying pickets had of course been thrown out from General Porter's force, now posted to keep the advancing rebels at bay until the still immense trains of stores and ammunition could be conveyed to Harrison's Landing, and the siege-guns and field-batteries placed in position at Malvern Hill and other points guarding the new base. McClellan had evidently calculated upon making the last and effectual stand at Malvern Hill, and the rebels had quite as evidently calculated upon his doing so if allowed to reach it; and on the issue of the struggle in that neighborhood was to depend the question whether the Union forces were to be driven pell-mell into the James River, surrender or hold their own and repulse their assailants. Sudden attacks and attempts at surprise were naturally expected by the rear-guard at any moment; and against these usual and unusual precautions had been taken, which would have satisfied old Frederick himself--that hard-headed old soldier who dreaded nothing in war but an attack by surprise. The nature of the country in the neighborhood as well as indeed along the whole line from the Chickahominy to the James, abounding as it did in woods and swamps, made it impossible to form extended lines of battle even at the spot where successful defence and the holding of a certain position appeared to be the most necessary. Many regiments had not even room to deploy more than half the length of their proper fronts; and the full strength of the command could not possibly be brought to bear against an attacking foe, distributed as it was in knots for miles across the country. These natural obstacles, meanwhile, were not disadvantageous to the rebels. Their superior knowledge of the section, with its numerous minor swamp-roads, forest-paths and approaches necessarily unknown to the Union forces, gave them immense advantages, such as they had not been slow to improve, in corresponding circumstances, during the whole of the preceding campaign. Aware of these facts, a night attack on Monday might have been expected by the Federal officers, and the men had slept on their arms in anticipation of it. But White Oak Swamp had been too severe a trial of courage and energy; they were not disposed to attack again before receiving more of the reinforcements steadily pouring onward from Richmond; and as a consequence the wearied troops had been allowed to pass the night without disturbance, and they had even overhauled the remains of rations remaining in their haversacks and made their scanty and unsavory breakfasts, long before the expected hostile cloud burst upon them.