 McClellan 's army began to sail from Alexandria on March 17 . It was an armada that dwarfed all previous American expeditions , transporting 121 @,@ 500 men , 44 artillery batteries , 1 @,@ 150 wagons , over 15 @,@ 000 horses , and tons of equipment and supplies . An English observer remarked that it was the " stride of a giant . " The army 's advance from Fort Monroe up the Virginia Peninsula proved to be slow . McClellan 's plan for a rapid seizure of Yorktown was foiled when he discovered that the Confederates had fortified a line across the Peninsula , causing him to decide on a siege of the city , which required considerable preparation . 
