When published in 1975, Terrible Swift Sword was called the first monster game. It was an attempt to recreate Gettysburg with regimental sized units, and featured over 2000 counters, a new movement and combat system, and three full color maps. It was so large, it shipped in two of SPI's flat boxes. The game sold quite well and demonstrated there was a market for monster games like War In The West, the Europa series, and what is considered the largest and most unplayable of all monster games, Richard Berg's own Campaign for North Africa.
Previous to TSS, most land-based board games featured "locking ZOCs". Once you moved the cardboard counter that represented a brigade or division next to an enemy unit, it was stuck there until one or the other was eliminated. TSS combat was much more fluid, and represented the two phases of Civil War combat.
First, regiments would open fire at each other at ranges of from two to five hexes. (Two to thirty hexes for artillery.) Second, one side would charge into the other's hex, taking fire along the way, and a hand to hand combat table would be consulted to see which side routed.
Another innovation was the use of strength points. Printed on the counter was the size and strength of the unit in hundreds of men. As the unit took casualties, a separate cardboard chit was placed under it, to record attrition.
TSS was also the first popular board game to use morale and make gamers like it. A unit that broke could no longer use ranged fire, and instead engaged in a rapid retrograde motion. Broken units can be reorganized by a leader unit.
In the first edition rules, a morale check was made against the size of the unit. This led to historical inaccuracies like the Union's Iron Brigade, composed of several veteran 200-400 man regiments, running away from green Confederate units which had 700-800 man sized regiments.
The second edition solved this via increased paperwork. Each brigade and division was given a rating, its Brigade Combat Effectiveness (BCE). Unit casualties were marked it on the sheet. When the brigade reached its threshold number, the entire brigade routed.
The 2nd Edition ruleset was used for battles like Shiloh, Manassas (both First and Second), and Pea Ridge. The system, now called GBACW (Great Battles of the American Civil War), was introduced to Strategy and Tactics subscribers in issue #67, Stonewall (Kernstown.) The gaming system actually lived on after SPI's demise, being picked up by other publishers.
Terrible Swift Sword won Charles S. Roberts (the Charlie) awards for Best Tactical Game and Best Graphics. At Origins 87, Richard Berg was voted the Hall of Fame Award, aka the Clausewitz Award. Some of his other innovative games include
Sources:
http://www.richardhberg.com
http://www.militaryhistoryonline.com/wargaming/mingus/default.htm
my personal copy (not for sale!)
| 
