Under the classical definition of siege the attacker is required to surround a point or at least dominate all means of access into or out of the point. But from the military standpoint, the siege begins with an investment, in which the attacker works to isolate the target, be that a city or other position held by the enemy. In the Civil War context, with an investment complete, the attacker could formally demand the surrender. Often just the threat of investment could prompt the defender to flee.
Returning from his visit with Rear-Admiral John Dahlgren on the USS Harvest Moon, Major-General William T. Sherman had these formal terms on his mind. He felt, after the briefing from Major-General John Foster in regard to the Coosawhatchie sector, that he could make a case that investment had been achieved. All routes on the Georgia side of Savannah were held by Sherman’s men. Foster could, at least, bombard the Charleston & Savannah Railroad. That situation was the basis for the “on the map” argument that the Federals had an investment of Savannah.
Sherman spelled out his intent in messages to both Major-Generals Oliver O. Howard and Henry Slocum, his wing commanders. To Slocum, at around 2 p.m. on December 15, he wrote:
General Foster has 5,000 men near the Charleston railroad, north of Broad River, and near enough to the railroad to command it, so that he feels sure that cars cannot pass either way; but he has been unable to reach the railroad itself with his men, on account of the enemy’s force. The gun-boats and General Howard occupy all other avenues of approach to Savannah connecting with your right. Now, if you can close the Savannah River to navigation, and also get a force over the Savannah River to threaten in flank any dirt road leading out of Savannah, between the city and Coosawhatchie, the investment of the city will be complete and the enemy will have no escape.
Sherman went on to explain that he wanted to move up siege artillery as quickly as possible. By using the Savannah-Ogeechee Canal, the guns could be transported directly to the Left Wing’s position. In addition, Sherman wanted Slocum’s
…batteries, which are nearest to the city, prepared to execute the foregoing plans, and he wants you to write him in full to-night any ideas that may have been suggested by your closer observation of the ground in your immediate front….
Slocum took a queue from this message. Upon receipt at 5 p.m. he responded:
The heavy guns can be used to advantage in my front. From my extreme left I can shell the city with the 3-inch gun. I think I can safely place a force on the Carolina side of the river and gradually work my way opposite the city.
The artillery positions mentioned were in Brigadier-General John Geary’s sector. However, as of that date, while Geary reported the Confederate fired an average of 300 rounds per day, he’d restrained the response back to sharpshooting.
However, it was on the Savannah River itself that most of the activity of the day took place. Colonel Ezra Carman had slowly worked the 3rd Wisconsin Infantry across to Argyle Island. On December 15, Carman pushed over the 2nd Massachusetts.
This move gave the Federals firm possession of the island. Along with the two captured Confederate gunboats, this effectively sealed off the Savannah River upstream of Savannah, as intended by Sherman. Furthermore, the Federals had possession of stores of rice and several rice mills. But after going over to observe the progress himself, Carman was not satisfied with just rice. Already he had patrols across the river onto the South Carolina side, meeting little resistance. He saw an opportunity. The road leading north out of Savannah, the Union Causeway through the ricefields and marshes, seemed within reach.
Returning to the mainland, Carman went to press his case up the chain of command. At the 20th Corps headquarters he addressed both Brigadier-General Alpheus S. Williams and Slocum. Impressed, Slocum agreed to reinforce the effort with the rest of Carman’s brigade and Battery I, 1st New York Artillery. He also sent a report to Sherman at 9 p.m.:
I have two regiments on the Carolina shore north of Clydesdale Creek. To-morrow morning the remainder of the brigade, three additional regiments will endeavor to take the line from Clydesdale Creek to a point on the Savannah River opposite to Cruger’s Island, with orders to intrench on that line and feel forward toward the causeway road. With your consent I will try to place a division on the line marked 2 on the inclosed diagram. It will be necessary to move with some caution on that side; and, to render the position entirely safe, it may be necessary to throw an entire corps over, with instructions to intrench strongly.
Here is the map Slocum attached:
Clearly Slocum was ready to jump in with both boots. If he followed by moving the Twentieth Corps, Howard would need to shift the right wing to the north as compensation. But Slocum did practice some self restraint:
I shall go no further than to send a brigade over to take the line marked 1 until I hear from you; but I have no fear of placing a corps on that side; and this done the fate of the city is sealed.
Slocum did relate one concern. The Confederates has, as of the day before, moved a gunboat up the Back River (to a point indicated as “B” on his map) to shell the Federal positions. The Federal guns could not reply to this.
There is one additional point which escaped Slocum’s or Carman’s assessment (even when Carman detailed these events post-war). The Confederates maintained several positions along the Union Causeway. These were batteries erected years earlier fearing threats from Hilton Head. While not substantial fortifications, these were sufficient to control the narrow means of access to the causeway. Not to say these were unassailable. But that is to say the effort would require detailed planning (of the sort that brought success at Fort McAllister).
Sherman received Slocum’s note at around 11 p.m. In a response sent shortly after, he authorized the movement, but limited to only one brigade,
… and instead of threatening south toward the Union Causeway, rather let it threaten eastward toward the road marked as running up toward Augusta on the east side of the Savannah River, seemingly threatening in flank the movement of troops attempting to escape from Savannah.
Sherman promised to explain his intent in person the next day. But, “A messenger is just arrived from General [Ulysses S.] Grant with dispatches of importance.” The message, carried by Colonel Orville Babcock, began:
On reflection since sending my letter by the hands of Lieutenant [William] Dunn* I have concluded that the most important operation toward closing the rebellion will be to close out Lee and his army. You have now destroyed the roads of the South, so that it will probably take three months, without interruption, to re-establish a through line from east to west. In that time I think the job here will be effectually completed. My idea now, then, is that you establish a base on the sea-coast, fortify, and leave in it all your artillery and cavalry, and enough infantry to protect them, and, at the same time, so threaten the interior that the militia of the South will have to be kept at home. With the balance of your command come here by water with all dispatch.
Grant indicated Sherman was to move north in person to command his forces. The message was sent in context of the rising frustrations with Major-General George Thomas at Nashville. Though he left some room to be convinced otherwise, Grant wanted Sherman close to him for a final push in Virginia.
With Grant’s message, Sherman faced an entirely new situation. The armies that had arrived outside Savannah could not become engaged in siege operations, lest they become pinned down. The last things these new orders would allow would be a flanking operation into South Carolina. More than anything the Confederates could do, the change in tone from the General-in-Chief put a damper on what Carman might do north of the city.
Note – The message sent by way of Lieutenant Dunn was that awaiting Sherman at Hilton Head, sent on December 3 and received on December 14 while with Foster and Dahlgren. The message sent by way of Babcock was sent on December 6 and as mentioned above, received on December 15.
(Citations from OR, Series I, Volume 44, Serial 92, pages 636 and 718-21.)