Northern publishers produced envelopes that commemorated successful battles waged during the war. Arguably the most important naval battle of the Civil War, the Union’s USS Monitor and the Confederate’s CSS Virginia (built from the remnants of the USS Merrimack) took place on March 8-9, 1862. Other envelopes depicted maps referring to battles, such as the Battle of Shiloh, a two-day battle fought in southwestern Tennessee in April 1862. Although considered a Union victory, the battle's cost was high. Union forces suffered 13,000 casualties, compared to 11,000 Confederate ones. Another envelope, depicting soldiers storming the ramparts of a fort does not appear to refer to a particular battle. One observer has noted that the uniforms suggest that this may have been an illustration from the earlier Mexican War (1846-1848) that was re-used for this Civil War era envelope.