A major element of Sherman's march to the sea was his army's burning southern farms and cities. The rationale for what Sherman called "Total War," approved by Grant and Lincoln, was that it would cause rebel troops to desert their units to check on family and home, and "break" the spirit of the Confederacy. Like the rationale for the atomic bomb, it was argued that the tactic would shorten the war and ultimately save lives? Do today's historians defend Sherman's tactic? Wouldn't it today be a war crime and a crime against humanity notwithstaning? An acceptable answer will describe what today's historians are saying about Sherman's march in light of 21st century attitudes about war. I DO NOT want opinion-based answers!
Sherman burned parts of Atlanta - the military stores area and destroyed its railroads to prevent it being used as a base against him. As the city was defended by an army, this is not improper by the rules of war. He allowed civilians to leave to the South, or be carried by rail North. Note that large parts of the city were burned by the retreating Southern Armies during the retreat, and the Union did not advance until the next day.
Note that uncontrolled fires as a Southern Army retreated before the US moved in also happened by Lee in Richmond.
During the march through Georgia, few structures were burned. Any government stores and railroads. I believe plantation cotton gins were usually burned, as this supported the government through the blockade. I can't recall a town burned.
The March through South Carolina was much more free with the burning along the march, although the usual Civil War standard was that empty buildings were more likely to be looted or torched than if someone stayed home to watch the place.
The Burning of Colombia is the other incident and there again we have a similar situation to Atlanta - Wade Hampton's Cavalry burned some stores on the way out, there was a windstorm that blew up the flames, and deserters from both sides seem to have gotten into some liquor and possibly set some more fires too. After a while, US troops secured the city and helped fight the fires and save some buildings. While some say Sherman burned the town, knowing his character if he had ordered it burnt, he would have done so and crowed about it. So I believe his story that it was an accidental act, with a bit of his blaming Wade Hampton for political effect.
When the army left South Carolina, they were back on good behavior for the rest of the campaign.
So, basically, I don't think that the idea that Sherman burnt towns during his marches stands scrutiny at all, much less as a tactic. His tactic was to move freely, subsist for food and forage on the locals (allowable by the laws of war), and destroy government property. The fact that he could not be stopped punctured the morale of the CSA.
As far as modern international law is concerned, things get interestingly complicated.
There is in fact a Geneva Convention protocol that was specifically written in 1977 to outlaw "scorched earth" strategies like (the popular view of) Sherman's March. However, the United States has not ratified that protocol. So current US policy seems to be that such actions are within bounds. Thus any southerner who wants to argue about modern international law being violated in Sherman's march probably needs to have a talk with their congressman first.
The effect of the march, if not decisive, was still a huge blow. Farms and industry in a wide swath of the state were destroyed, and railroads were ripped up to prevent nearly all movement of goods and men to support Lee's army. Since the Union already had the Confederate ports blockaded, the above two actions not only knocked Georgia completely out of the war, but also cut off any states west of Georgia. Effectively, thanks to Sherman's march, the Confederacy was now down to just the Carolinas and Virginia. (In the spring, he proceeded up through the Carolinas).
Sherman's March to the Sea (and "total war") was a logical extension of the so-called Anaconda Plan for beating the Confederacy. As the Anaconda Plan itself was "rehabilitated" by modern historians, so was Sherman's extension of it.
The original Anaconda Plan was seen as a "backup" plan to the main plan of capturing Richmond, and ultimately Virginia, one of the richest, most populous, and best-defended states in the Confederacy. The idea was to "break the enemy's strength."
The purpose of the Anaconda Plan was to "hit the enemy where he is weak," along the Mississippi River, where the South concentrated fewer, lower quality troops, and had no gunboats to speak of. The plan was to capture those parts of Tennessee and Mississippi, along the Mississippi River. If successful, the Union would occupy two out of eleven Confederate states, and isolate three more (Texas, Arkansas, and Louisiana), effectively cutting the main Confederacy in half.
The remaining six Confederate states were Virginia, North and South Carolina, Georgia, Florida, and Alabama. After Sherman captured Atlanta, he devised a daring plan to march east to the Savannah River (the border between Georgia and South Carolina), and downriver to Savannah itself. The idea was to "lop off" three more Confederate states (Georgia, Florida, and Alabama) from Virginia and the Carolinas. This was the infamous "March to the Sea."
It was a daring plan that required a form of "total war." 1) Because his supply lines back to the Mississippi River was menaced by Hood's defeated forces in Tennessee, Sherman had to "let them go" and live off the land (as Grant did in brief campaign against Jackson, Mississippi prior to Vicksburg), which is allowed by the rules of war. 2) As Sherman marched through Georgia, he systematically freed the slaves in his path, in line with the Emancipation Proclamation, which Southerners considered a form of "terrorism." (Many slaves marched behind Sherman's army, adding to the devastation.) 3) Sherman systematically destroyed Georgia's manufacturing capacity, making a point of removing and twisting rails from southern railroads, as far as his troops could march, to hinder the South's recovery. Sherman did make a point of sparing civilians that were judged totally "innocent," but anyone who helped the South in any way (e.g. manufacturing or shipping supplies) was despoiled.
This probably shortened the war because the weak remaining Southern forces couldn't counteract Sherman. It might have lengthened the war if Hood had returned from Tennessee and caught Sherman in the campaign of mutual destruction that he began at Atlanta. But by the time of Appomatix, the South had lost Virginia to Grant, and South Carolina to Sherman, meaning that essentially, only North Carolina was left to Confederacy, even if Lee had managed to escape.