Oh, where to begin?
I think I may be the person referred to that brought up the statement that Missouri lost 1/3 of it's population during the war. I first learned this via a You Tube video that author/historian T. J. Stiles made about in his book "Jesse James, Last Rebel of the Civil War." But after re-reading his book a year ago or so I came upon the same statement in his book, that I missed in my previous readings. On Page 156, Stiles says, "The population of Missouri fell by an estimated 300,000 people between 1861 and 1865. Roughly one out of every three citizens had been killed in battle, murdered at home, driven out by guerrilla threats, banished by authorities, or simply had fled to a more hopeful place." He backs this claim by footnote #20 where he gives credit to (quote), "Fellman, 242; Foner, 19; for Northern wartime expansion in general, see McPherson, 819-9. The state census of 1864 calculated a loss of 262,146, a number that certainly grew in the aftermath of Price's raid and subsequent Union countermeasures; James Fernando Ellis, "The Influences of Environment on the Settlement of Missouri (St, Louis: Webster Publishing, 1929), 144-5 (end quote).
The only book the Stiles mentions that I have in my library is Fellman's book, "Inside War," and on page 242 of that book, Fellman states, "When one considers the widespread depopulation of the war-ravaged countryside, which I would estimate as a loss of at least 300,000 by 1865, the population may nearly have doubled between 1865 and 1970." So now we know where Stiles got his 300,000 figure from. If one only looks at the increase in population from 1860 to 1870 we come up with an increase of approximately 45%, which is remarkable, but here is where mere statistics fail us. If one looks at the 1860 population of Missouri of 1,182,012 and subtract the war-time loss of the estimated 300,000 we have a state population of 882,012 at the war's end. In 1870 the population was 1,721,295, a gain of 839,283 persons from the estimated end-of-war 1865 population, a 71% gain. That is truly remarkable.
Where did the gain in population come from? According to Fellman, (pages 242-245), there was a baby boom after the war that accounted for 61% of the increase. But the most dramatic gain (71%), came from immigrants from the states of Illinois and Ohio, who made up 43% of the immigrants, with the states of Indiana, New York, and Pennsylvania making up the rest of the increase. By 1870, Missouri was a different state than it was in 1860. Northern born settlers had accounted for 38% of the population in 1860; by 1870 they made up nearly 58%. The number of foreign-born residents of the state had also increased by 1870, but as a percentage of the population, their numbers had decreased from 15% in 1860 to 13% in 1870; in St. Louis, a city where 60% of the population was foreign-born in 1860, they now made up 36% of the city's population by 1870. The driver of the population increase after the war was largely due to relatively cheap land, new railroads which opened up eastern markets, and an industrial boom primarily centered in St. Louis.
Turning to the area of Missouri which saw the most destruction from the war, the "Burnt District," an area of 2,200 square mile that was cleared of all rural and small town inhabitants due to Gen. Ewing's General order #11. The total population of this area was around 40,000 inhabitants pre-war, of which slightly over 30,000 rural people were forced to leave their farms and small towns with a ten day notice. The farms and small town they left were burned out and what they couldn't take with them were either burned or taken off to Kansas. The counties of the "Burnt District," consist of Jackson (where Kansas City is located), Cass, Bates and the northern townships of Vernon County. The northern border of Jackson County is on the Missouri River, and it's western border is on the state line with Kansas, the rest of the counties are located south of Jackson County in the order given. According to author Tom A Rafiner and his book "Cinder and Silence: A Chronicle of Missouri's Burnt District 1854-1870," after the end of the war, these counties opened up for repopulation with the majority of new "Heads of Households" being from Union states. For example, from Rafiners' book, pages 259-262, the Cass County Head-of-household count for 1860 was 1,526. In 1870 the birth-place for new head-of-households was 221 from Confederate States, 410 from Border States, 1,187 from Union States, and 200 from other sources, (foreign-born). For the entire "Burnt District," the 1870 growth in Heads-of Households increased by 80.5% from 1860, with Jackson County increasing by 77.3%, Cass County 81.3% and Bates county increasing by 88.6%. The birthplace of these Heads of Household were 1,491 from Ohio, 1,380 from Missouri, 893 from Kentucky, 724 from Pennsylvania, 691 from New York, 666 from Indiana, and 595 from Illinois.
Clearly, the increase in population of Missouri was dramatic, not only for the increase due to the native birth rate, and from Missourians moving back into the state, but from the huge increase in out-of-staters who moved into the state, and these were primarily from northern states. This immigration totally changed the state. Pre-war the state identified as being Southern; by 1870 it identified with other northern midwestern states in it's politics and economy.