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....