Good question and I wish I had more for you on it. There was a very dry history of Texas written in the 1960s that addresses this to some degree, but I neither have a copy nor can I recall the name of the work.

Turning to TR Ferenbach, he briefly touches on this in “Lone Star,” considered by many to be the best singular monograph on complete Texas history, and one used in many college Texas history classes.

Personally, I don’t care for this book, but none more comprehensive have become popular enough to become textbooks. Thus, this one is a “standard.”

At any rate, in one of the few actual updates made to the book between its 1968 and 2000 reissue, notes:

“The land shaped those who lived upon it more than they changed it. Hostile, yet with a beauty the second generation came to love, with crashing meteorological changes that punished man and beast, with wind that made them uneasy, yet volatile and free, it somehow arouse a sense of music in the Spanish-Mexican soul. In Americans, it made feelings they could not articulate.

The land, the climate, the sense of endlessness yet constant change made all who came there hospitable, patriotic, violent, and brave.” (Ferenbach, “Lone Star, pp. 724-725. Da Capo Press, 2000).

While Ferenbach cites this, in the 20th century, the reality is it was more indoctrination through public school education.

“Texas Public School Week,” (as it was formerly called) occurs in March, around the time of March 6, the Alamo anniversary. It was established in 1950 at the urging of the Masons. (SOURCE).

This week has served, for 70 years, as a time elementary grade children receive repeated education on the history of our state. Texas History is only taught as an independent class in fourth and seventh grades. TPSW basically created a week where all students participated in a lot of activities relating to Texas history.

Regrettably, there is little documentation on this other than the dozens of worksheets you can find online for teachers to use during this week—things like worksheets on the state flower, etc. however, I, my sister, and my stepmother as a teacher all participated in this for many years. It was a looked forward to tradition.

Personally, I would surmise that a Texas public school education is more likely responsible for indoctrination of this type of thought in the Baby Boomer, GenX, and potentially Millennials. However, I’m unaware of any academic research on this. I am familiar with it from having served on school district committees that adopted these textbooks, and from citizen lobbying the State Board of Education during the years they select textbooks for Texas history.

The way our school Texas history textbooks are written also aided in indoctrination of these thoughts. For the younger grades, these books very much boost Texas as a one of a kind place and Texans as a unique people.

The work of the Daughters of the Republic of Texas to promote enhanced teaching of Texas History in the 1930s and 1940s also played some role in this, I believe. Again, no historian has really looked at this, so there aren’t sources. DRT is the heraldry organization for descendants of people who came to Texas prior to it joining the U.S.

I truly wish there were more easily available sources for this answer.

I am happy to pick up what /u/vpltz put down:

Personally, I would surmise that a Texas public school education is more likely responsible for indoctrination of this type of thought ... The way our school Texas history textbooks are written also aided in indoctrination of these thoughts. For the younger grades, these books very much boost Texas as a one of a kind place and Texans as a unique people.

Texas holds a unique role in American education history. To be fair, every state has a remarkable and compelling history. But Texas... Texas, for a whole lot of reasons, exerts its own gravity - both on the country writ large and the residents of the state. Part of that comes from the fact that unlike most states on the East and West coasts which constructed a state system out of a patchwork of schoolhouses and districts, Texas developed a state system following the Civil War. What this means in a practical sense is in 1903, when the Texas legislature created a structure to ensure uniform textbooks across the state, they caught the eye of every company looking to sell anything to schools. I get into what that meant for textbooks here.

In a post from last year, I used Texas as an example to answer a question about differences between how history is taught in the South versus the North. I won't copy the whole thing as it was fairly long but there a few points I want to highlight.

[After an overview of how New York State approaches history education] So, to summarize with a broad generalization, Northern states - through a combination of tradition, guardrails around course expectations, and cultural norms - expect(ed) districts to provide students with a comprehensive history education that focuses on America's shared history, which includes the Civil War.

[Afer an overview of how Texas approaches history education] So, to summarize with a broad generalization, Southern states - through a combination of tradition, textbook adoption, and cultural norms expect(ed) teachers to provide students a history education that focuses on a comprehensive understanding of what it means to be a Southerner, an identity framed around the Civil War.

Which is to say, Texas educators deliberately built the creation of a Texas identity into the curriculum from the get-go. A lot of that was pushed by the United Daughters of the Confederacy, who were incredibly active in the state. They did curriculum audits, provided free books to libraries, pushed for the valorization of Texan men who died in the war, and campaigned to get the men they wanted onto the Texas Education Agency.

The basic gist is that adults in Texas, starting shortly after the Civil War, felt it was important for Texas children to develop a sense of what it means to be a Texan and used various levers in education to make that happen.