George B. McClellan was a second lieutenant in the formation of combat engineers that accompanied Maj. Gen. Winfield Scott's army of invasion during the Mexican War (1846 -- 1848). His diary and correspondence written during this period records a rich record of the campaign and offers unique insights into the character of his fellow Engineers; the friction that arose between professional soldiers, officers and men of the volunteer regiments that made up Scott's command; and much about the character of "the young Napoleon," reflecting the talent, the ambition, and the arrogance that characterized the engineer, businessman, soldier, and future politician.