Please provide some perspective on the battle at Gettysburg in terms of some of the modern warfighting principles, such as 'economy of force' , 'surprise', 'simplicity', and/or 'unity of command'.
not necessarily all of them, just those that may have applied significantly to the battle - and more specifically to the union army.
I will address the issue of surprise first because, as the battle unfolded, this was a crucial element in the success of the Federal Army.
As Lee and the Army of Northern Virginia moved north into Pennsylvania during the last week of June, 1863, Lee's "eyes and ears," JEB Stuart and his cavalry, was north and east of Gettysburg causing a lot of fear in the North but not doing its job of reconnaissance, the result being that Lee did not know where George Meade and the Army of the Potomac were or what their strength was. When Confederate General Henry Heth moved into Gettysburg on July 1 looking for, among other things, shoes, he thought he was facing a small unit of John Buford's Federal cavalry, which he was, but did not realize the entire Federal army was within a few hours' march away from Gettysburg. The Federal Army had inadvertently achieved surprised, not because of action on its part, but on the inaction of one of Lee's best generals, Stuart.
After the Confederates pushed the Federals out of Gettysburg on July 2, and George Meade arrived with most of the Army of the Potomac, Meade made a very simple tactical decision to occupy the "high ground" just outside Gettysburg proper, the now-famous Cemetery Ridge, as far south as a small mountain known as Little Round Top, a commanding height at the far end of the Federal line. From a tactical and strategic perspective, Meade and the Federals commanded an almost perfect defensive line.
One of the problems facing the Army of the Potomac (actually, all Federal armies) is that many of its commanders were political appointees, and many commanders were of two factions--abolitionists and those who were pro-slavery. These two groups of officers, especially at brigade command and higher, often did not cooperate fully even in the midst of battles. Gettysburg was no exception. In one incident on July 2, General Dan Sickles, who was a political enemy of his commander, George Meade, moved a brigade out of position in the middle of an assault by Confederates, jeopardizing the integrity of the entire Federal line and exposing Federal troops to his right and left to Confederate fire. This is probably the greatest violation of "unity of command" during the battle. On the other hand, an interesting, if somewhat controversial, aspect of Meade's idea of command unity is that, during a conference with his generals on the evening of July 2, Meade, rather than telling them what he wanted to do, called for vote of consensus on Federal action for July 3. In any event, Meade made very good use of subordinates during the battle, especially General Winfield Hancock, delegating much authority for directing Federal activities on the field.
From an "economy of force" perspective, Meade is credited with the decision to keep the Federal Army essentially in its defensive positions on Cemetery Ridge, knowing that Lee, who believed his troops could achieve anything, would attack the Federals even in a strong defensive position. The resulting attack by Confederate General George Pickett's troops, famously known as "Pickett's Charge," resulted in a tactical standoff on July 3 but a profound strategic victory for the Federal Army because so many of Pickett's veteran troops were either killed, wounded, or captured.
In sum, then, although the Federal Army has its usual problems at Gettysburg, Meade and other commanders employed relatively simple battle plans, achieved at least a workable level of command unity (with exceptions, such as Sickles), and, most important, allowed the Army of Northern Virginia to break itself on the Federal Army's incredibly strong defensive position.