Spending most of the summer at home, he was able to rejoin the army early in September, when Lee began his daring invasion of the North,--a political even more than a military move. Then Confederate audacity was fully matched by Pennsylvania's patriotism. Although the State had already one hundred and fifty regiments in service, Governor Andrew D. Curtin called for fifty thousand more men. Within ten days that number of militia were armed and equipped, and in the field. Millionaires and wage-earners, professors and students, ministers and their congregations were in line guarding the Cumberland Valley. Neither disasters nor the incapacity of generals chilled the fierce resolve of Pennsylvania's sons, who were determined to show that the North could not be successfully invaded, even by veterans led by the bravest and most competent generals of the age. Carleton was in the saddle as soon as he learned that Lee had moved. From Parkton to Hanover Junction, to Westminster, to Harrisburg, to Green Castle, to Hagerstown, to Keitisville he rode, and at these places he wrote, hoping to be in at the mightiest battle which, until this time, had ever been fought on American soil. For many days it was a mystery to the Washington authorities, and to the Army of the Potomac, where Lee and his divisions were; but, with his usual good fortune, Carleton was but nine miles distant, at Hagerstown, when the booming of the cannon at Antietam roused him from his sleep. It was not many minutes before he was in saddle and away. Instead of the ride down the Sharpsburg pike that would have brought him in rear of the enemy, he rode down the Boonsboro road, reaching the right wing of the Union army just as Hooker was pushing his columns into position. Striking off from the main road, through fields and farms, he came to Antietam creek. He found a ford, and reached a pathway where a line of wagons loaded with the wounded was winding down the slope. On the fields above was a squadron of cavalry to hold back stragglers. In the first ambulance he descried a silver star, and saw the face of the brave General Richardson, dead, with a bullet through his breast. At the farmhouses, rows of men were already lying in the straw, waiting their turn at the surgeon's hands, while long lines of men were bringing the fallen on stretchers. With hatred of war in his heart, but with faith in its stern necessity, Carleton rode on to see the fight which raged in front of Sumner, noticing that the cannon of Hooker and Mansfield were silent, cooling their lips after the morning's fever. Of the superb Pennsylvania Reserve Corps, which he had seen a year ago at review, there was now but a remnant. He ascended the ridge, where thirty pieces of cannon were every moment emptying their black mouths of fire and iron.