Before the first shots were fired at Gettysburg - for many, the most significant engagement of the American Civil War - a private battle had been raging for weeks. As the Confederate Army marched into Union territory, the Federal Forces desperately sought to hunt them down before they struck at any of the great cities of the North. Whoever could secure accurate information on their opponent would have a decisive advantage once the fighting started. When the two armies finally met on the morning of 1 July 1863 their understanding of the prevailing situation could not have been more different. While the Rebel Third Corps was expecting to brush away a group of local militia guarding the town, the Federal I Corps was preparing itself for a major battle. For three brutal days, the Rebel Army smashed at the Union troops, without success. The illustrious Confederate General Robert E. Lee would lose a third of his army and the tide of the rebellion would begin its retreat.