Shiloh is a popular battle to wargame, so it's no surprise it's included in the Blue & Gray quad by SPI and the second edition from Decision Games.
It was a tough battle for both sides, but particularly for the Confederates. They sacrificed a lot of strategic position in order to mass a large army to attack Grant, achieved complete surprise and yet, in the end, fell short.
The situation is a challenging one for the CSA. The Rebel player has to make the most of his initial advantage. This requires an efficient and energetic offensive and a little bit of luck as well.
The Rebel army starts massed on the field, with 109 factors of infantry, 18 factors of artillery and 5 factors of cavalry. It faces a Federal Army of 97 infantry factors, 21 artillery and 3 cavalry, so the CSA has only a marginal edge in strength.
But the Federal force is under severe movement restrictions for the first two turns, barely able to move and only in restricted directions, so the Confederates have an opportunity to mass and maneuver for maximum advantage.
Time is of the essence, however. Buell's Army of the Ohio brings another 44 infantry and 4 artillery factors into the battle across the river starting on turn 4. Wallace's 3rd Division appears behind Grant's lines with 15 infantry, 2 artillery and 1 cavalry factor around the same time. And the Navy contributes 3 factors of indestructible artillery fire from its two gunboats from the river.
That this imbalance in forces is tough to overcome is borne out in statistics from online game-playing service Hexwar.com, which offers Shiloh as one of its games. It has the game in both a "classic" version (without the attacker effectiveness rule) and standard (with the rule). Either way it's a tough game for the Rebels. Under the classic rules Grant's Federal army has won 296 times compared to 136 CSA wins and a single draw as of Jan. 22, 2008. The standard game doesn't change things. Under those rules the Union has won 897 games, the Confederates 463 and there have been four draws.
I think the game is a little closer than the statistics indicate because those figures necessarily include a lot of games by inexperienced players and the CSA side is much harder to play at first. With experience the CSA players understand what they must to to stay competitive and games become closer, although it's still harder for them. There's little margin for error or bad luck for the CSA.
The game basically goes through two or three phases.
The first phase comprises turns 1 and 2. During these turns the Union player must move any unit that's not in a Confederate zone of control one hex north or northeast. This makes it hard for them to form an effective line and gives the Rebels a good opportunity to set up favorable attacks. There are many ways to skin the cat, but in general the Confederate player must do significant damage to the Union army and set up for Phase 2 during these two turns. Letting the Union player slip away at this point is an unrecoverable error.
Phase 2 usually comprises the balance of the turns before nightfall, although it sometimes can last until the first turn or two of the next day. During this period the Confederate player must turn all his efforts to driving for Pittsburgh Landing. Winning the game requires capturing that point, not just for meeting the victory conditions, but also for blocking the Army of the Ohio from crossing the river. It's really and all-or-nothing situation. While it's theoretically possible for the CSA to win a marginal victory without holding the Landing by having a 2-1 advantage in losses inflicted, in practice this is not achievable if the Army of the Ohio is allowed to get across the river.
The game may be essentially over with Phase 2 if the CSA succeeds in capturing the Landing. Reaching that point necessarily means most of Grant's army has been destroyed and if Buell's troops can't cross it's usually possible for the Confederates to mop up the remnants.
The most difficult decision for the Confederates is when and whether to transition to Phase 3. This is the point at which the CSA realizes it's not going to take the Landing hex. This almost certainly means the Confederates will have to accept some level of defeat, but a properly timed and conducted transition to Phase 3 can keep the Union victory level down, which may matter if you're playing a two-game match.
Pushing too hard and falling short anyway will mean that the Union troops are well poised to make a devastating counteroffensive that can come close to wiping out the Rebel army. Letting up too early, of course, means that Confederates won't win at all. This decision is a delicate one and experience is the best guide, but as a general rule the Confederate player should know by the time Turn 6 starts whether the landing can fall or not. If not, it's time to cut losses and prepare for Phase 3. Nightfall can be used to pull back and set up for a delaying action over the final six turns that will make it hard for the Federals to inflict enough losses to get more than a marginal victory.