General Sherman cut all three of the main railroad lines into Atlanta. When they got the third railroad line, Southern forces gave the order to evacuate which means to leave. 
The battles to take control of Atlanta were called the Atlanta Campaign. General William T. Sherman was the general for the Union forces. General Joseph E. Johnston was the general for the Southern forces at first. It took time to defeat Atlanta, many months.
General Sherman successfully attacked and defeated the Southern forces in Chattanooga, Tennessee. Next, he moved his armies through North Georgia. And then, he moved south to Atlanta. Four things helped the Union to be able to do this. They had a good supply line. They were good at keeping their soldiers fed, clothed, and armed. They kept the supplies coming. Second, they had gained confidence from winning battles in Tennessee. Third, they had more soldiers. There were about twice as many Union soldiers. And fourth, General Sherman was very rough and tough. General Johnston, for the South, was not so forceful.
When they got to Atlanta, Sherman’s troops found a well-prepared city. For a year Atlanta had been fortified with trenches, cannons, troops, and barriers. Atlanta had been building a defensive line. It was eleven miles long and well dug in. Sherman knew a direct attack would be foolish. He would carry out a siege. He also did two other things. He shelled and flanked Atlanta. The plan was to try to force the people to leave Atlanta. They shelled the city for five weeks. There were 3,000 residents in the city. About 20 of those who lived in the city were killed. Union troops also did a flanking move. They made a move to the side, around the east side, of the city. Going to the side was part of his plan. Sherman wanted to cut off the railroad lines. That would cut southern supply lines. They tried to force the city to give up Atlanta. The Siege on Atlanta lasted five months.
The Union wanted to destroy the ability of Confederates to fight the war. That was their military goal. One way to do that was to destroy the railroad lines.
General Sherman cut all three of the main railroad lines into Atlanta. When they got the third railroad line, Southern forces gave the order to evacuate which means to leave.
Another thing Union forces wanted to do was to get rid of the South’s textile industry. Just outside of Atlanta there were several mills. The New Manchester Mill was located 13 miles west of Atlanta in Lithia Springs. It was on the edge of Sweetwater Creek. It was a textile mill. At one time it produced 750 pounds of textiles a day. They made clothes and rope. It was one of the largest factories in Georgia.
Early in July, 1864, Major Thompkins destroyed the woolen mills in Roswell, GA. He then ordered that the New Manchester Mill be burned. They poured flammable material on the floors and lit it on fire. Mill workers that worked there were taken to prison in Louisville, Kentucky. Union forces made them sign a paper to pledge allegiance to the Union. Then, they sent them across the Ohio River and told them not to return to Georgia until the war was over.
There are some interesting stories about that area where the mill was in Lithia Springs. There was a general store near the mill. It was a company store operated by Nathaniel Humphries. It was rumored that Mr. Humphries and his brothers manufactured shoes for the Confederacy in the basement of the store.
The other story we can learn when we go down to the old mill site. Turtles live in the area surrounding the New Manchester Mill ruins. Favorite logs may be crowded with turtles. Some, or many, of them may be piled on top of each other. Can you tell if they are cooters or sliders? Cooters have yellow stripes on the head and Sliders have red ears.
On September 2, Sherman's troops entered and took control of Atlanta. They stayed two and one-half months. Union troops were ordered to destroy factories, shops, foundries, and train stations. They wanted to stop the Confederates from continuing the war. Destroying Atlanta helped the North to defeat the South. It had been the second largest city. It was very important in supplying things for the war.
Sherman’s main goal was to quickly end the war. He did it with the idea of total war. This meant to not just win battles but to also show people that he meant total business. However, he did not burn Atlanta to the ground. He moved citizens out of town. He destroyed the city’s ability to make war. In addition, one-third of the city was ruined because of the trenches they dug and built to protect themselves. It was also General Hood, from the South, that blew up large amounts of ammunition, train cars, and a roller mill. He did this as they were leaving.
The Atlanta Campaign was very important. The results helped the North. On the other hand, they really hurt the South.
Text Credits:
Sweetwater Creek State Park, museum, and actual mill site for the Sweetwater Manufacturing Company (later the New Manchester Manufacturing Company)- Construction had been finished in December of 1840;
http://www.georgiaencyclopedia.org/file/12136;
http://www.georgiaencyclopedia.org/articles/history-archaeology/atlanta-campaign;
Sherman’s Total warfare- https://www.civilwar.org/learn/articles/scorched-earth;