It will be many years before a complete history of the operations of the armies of the Union can be written; but that is not a sufficient reason why historical pictures may not now be painted from such materials as have come to hand. This volume, therefore, is a sketch of the operations of the Army of the Potomac from August, 1861, to November, 1862, while commanded by General McClellan. To avoid detail, the organization of the army is given in an Appendix. It has not been possible, in a book of this size, to give the movements of regiments; but the narrative has been limited to the operations of brigades and divisions. It will be comparatively easy, however, for the reader to ascertain the general position of any regiment in the different battles, by consulting the Appendix in connection with the narrative.