Having previously commented on the fact that all of us to some extent are fortress-builders , I would like to take today to comment on one of the weaknesses about building fortresses alongside rivers. Of course, rivers are classic communications and supply routes, and the untrained military thinker might think that interdicting such obvious supply and invasion routes with fortresses might be a good idea, until one learns a wise lesson from the American Civil War, that is .
In his recent post on the Emerging Civil War History blog, historian Zac Cowsert comments on how the Southern fortresses in the Western Theater were death traps. 20,000 rebels were captured at Fort Donelson, another 7,000 at Island #10, another 10,000 at Port Hudson and over 20,000 at Vicksburg. All told, the South lost about 60,000 captured troops after sieges to massive river fortresses, troops that the South could ill-afford to lose. What was the problem with their strategy of holding highly defensible points (well, except for Fort Henry) alongside vitally strategic rivers? And what can we learn from their failure?
We can note in the beginning that there are many similarities in the various incidents which led to the garrisons becoming deathtraps. For one, the importance of the rivers guarded (the Tennessee, Cumberland, and Mississippi) drew large and well-provisioned Union invaders. In all cases, after an initial attempt at a forward defense or attack against the besieging forces (at Fort Donelson and at Champion’s Hill outside of Vicksburg), the rebel forces soon retreated into their fortifications and mounted a passive defense. The strength of that defense (such as at Port Hudson and Vicksburg) prevented victory by storming, but the Union was patient enough in all occasions to win the siege given their massive logistical advantages.
And it was indeed logistics that proved to be the biggest problem faced by these Southern deathtraps. Island #7 in the Mississippi only had one way in to supply, and with Union control of the river due to the weakness of the rebel riverine fleet the defenses could only survive as long as their tenuous connection to shore supplies lasted, and when that failed John Pope (!) got to become a military hero for a fairly straightforward logistical victory, at least until the disaster at Second Bull Run. Fort Henry failed because its siting prevented the fort from being able to defend from the flooding river, much less a river fleet. No army was needed to take that “fortress” in name only. Fort Donelson failed because of a lack of nerve among most of its defenders, leaving an honorable man to surrender after cowardly generals higher up had bungled its defense.
It is, however, the failures of Vicksburg and Port Hudson that are the most instructive. The Union drive to sever Confederate control over the Mississippi River was a decisive element of the eventual Union victory, allowing Midwestern states unimpeded trade to the Gulf of Mexico and cutting the Confederacy in two, preventing food and cotton from the Trans-Mississippi regions of the Kirbysmithdom from reaching the core of the Confederacy in the East. Both fortresses were well-defended with sizable garrisons (especially Vicksburg). Both fell within a matter of a few days in July 1863. Both failures were primarily logistical ones. The key advantages of the North over the South were in strategy and logistics, and in particular the aim of using logistical advantages as an element of strategy.
And that is what happened in Port Hudson and Vicksburg. Both cities were well-defended but static defenses isolated from any kind of logistical support. Once the sieges began there was no effort (except for a belated and ineffective effort on the part of Joseph Johnston near Jackson, Mississippi) to counteract the sieges. The large size of the garrisons and the inability of the South to supply the garrisons meant that Southern fortresses became traps for defenders who then became Northern prisoners of war.
That’s what went wrong–the garrisons of the Southern fortresses were “attractive nuisances” that drew powerful Union armies to attack necessary ground so that the North would be free to send supplies and troops along key river routes that were not powerful enough to prevent themselves from being surrounded and that had too many troops to survive a long siege (the more troops are in a garrison, the more supplies one needs for the same length of a siege). Without a more active army to counterattack the besieging forces, the garrisons were hopeless in the absence of large supplies. And the South never did solve its logistical shortcomings, making its fortresses militarily useless.
So, what lessons can we learn from this failure. For one, we need to understand that a fortress is not only a strong point but it is also a garrison that needs to be supplied. Isolated garrisons of strongpoints will only endure so long as there is enough food and water for them to hold on against forces. Without logistical capabilities, a fortress is merely a prison for its defenders (like Jerusalem against the Babylonians or Romans). Therefore, any person or nation that wishes to use strongholds for defense must pay great attention to ensuring the supplies of those strongholds as well as ways to counterattack the forces that will be drawn to attack those strongholds. A failure in either supplies or effective counterattacks makes strongholds largely useless, to be either bypassed, besieged, or stormed as the situation warrants.