Gun barrels glistening in the fine spring weather, the advancing columns of blue-clad troops made up the largest army America had ever seen, and - after months of training - it wanted to show its mettle.
As the day wore on, however, that confidence began to unravel with the discovery of a fortification so formidable it might have stopped an entire division had it been defended. More doubts cropped up in the next day's heavy rain, when both the Great Warwick and York-Hampton roads turned into morasses.
Just before noon, the advance elements of the Union left flank found their path blocked by the Warwick River, which - contrary to their disastrously inaccurate maps - ran east and west instead of parallel to the James River. And behind its banks loomed a deadly surprise - 1,800 Confederates swarming like hornets atop heavily entrenched, 40-foot-high bluffs - their flags flying and their cannon blazing.
So fierce was the fire from Lee's Mill that artilleryman William B. Jones - the Warwick County clerk of court - became known as "Hell Cat Billy."
And so stunned was the Union army - which stopped dead in its tracks - that one British observer who had described its advance as the "stride of a giant" began comparing it to "that of a dwarf."
"The earthworks along the Warwick River were a complete surprise. Nobody expected them," says West Point Museum curator Leslie D. Jensen, who explored the campaign in his book "32nd Virginia Infantry."
"And the Union's hesitation probably added a couple of years to the war."
Even 150 years later, there's no mistaking the surprise and awe caused by this epic 12-mile-long roadblock, which Confederate Maj. Gen. John B. Magruder erected to protect the back door to Richmond.
"Magruder is in a strongly fortified position behind the Warwick River, the fords to which have been destroyed by dams, and the approaches to which are through dense forests, swamps and marshes," Brig. Gen. Erasmus D. Keyes reported.
"No part of this line as discovered can be taken without an enormous waste of life."
Chief engineer Brig. Gen. John G. Barnard agreed, saying "the line is certainly one of the most extensive known to modern times." And as Union troops fanned out to probe along the river, they found a great chain of rifle pits, communication trenches and covered ways anchored by lethal gun positions even more imposing than Lee's Mill.
Adding to the impenetrability of this massive wall was a series of old mill ponds and new dams that protected two-thirds of the line with water obstacles.
"The Warwick River grows worse the more you look at it," wrote Union commander Maj. Gen. George B. McClellan, whose 1855-56 tour of the Crimean War provided grim insight into the killing power of such entrenchments.
"He's seen what these kind of positions can do to slaughter an army, so he's hesitant to order to an assault," says J. Michael Moore, curator of Lee Hall Mansion and co-author of the upcoming book "Drums Along the Warwick."
"But he believes in his artillery. That's the lesson he learned in Sebastopol. And he starts to bring up the big siege guns he'd planned to use on Richmond."
Much of McClellan's fateful delay is caused by his gross miscalculation of his enemy's numbers.
Both faulty intelligence and his by-the-book, mile-by-mile analysis of the troops needed to defend such an extensive line resulted in estimates of about 100,000 rebel soldiers.