Kenedy County, among the last Texas counties formed, was not established until 1921, when Willacy, Cameron, and Hidalgo counties were reorganized. The stated reason for the county's formation was the considerable distance to the county seats of the other counties. But perhaps more important was the attempt of ranching interests to stave off the growing power of farmers who were beginning to develop the Rio Grande valley. The new county seat was established at Sarita, where John G. Kenedy, son of the founder of the King Ranch, had built his headquarters. -Handbook of Texas Online: Kenedy County

Here's more:

Kenedy County is one of those peculiar counties created at the behest of wealthy ranchers (in this case, the King Ranch) so that the county can be run as a sort of fiefdom. About 400 people live in the county, most of them in the little town of Sarita. Only one paved highway enters the county: US-77, which makes a straight shot north-south. Aside from the few roads in Sarita, the entire county is essentially company land, and access is not permitted anywhere without permission.

So is it made up of precious farmland? Hardly.

Less than 1 percent of the county is considered prime farmland.

In the 1870's the Anglos moved in on the Mexican ranches. There were "skinning wars." Guess who won?

As to the formation of the county:

The stated reason for the county's formation was the considerable distance to the county seats of the other counties. But perhaps more important was the attempt of ranching interests to stave off the growing power of farmers who were beginning to develop the Rio Grande valley. The new county seat was established at Sarita, where John G. Kenedy, son of the founder of the King Ranch, had built his headquarters. Since that time the county has changed little. Although Kenedy County was a ranching area from the advent of the Spanish to the early 1990s, there have never been more than twenty-five ranches in the county, and most of the land still remains in the hands of the Armstrong, King, Kenedy, and Yturria interests. (my emphasis)

Then came the oil companies. The county's ranches, which by 1945 numbered only 7, leased land for oil production:

Oil was discovered in the county in 1947, and in the early 1990s oil and natural gas accounted for the largest source of nonfarm earnings. Production of crude oil in 1990 was 643,446 barrels; between 1947 and January 1, 1991, a total of 31,800,494 barrels was produced.

How about the workers in the county?

...only 15 percent of the population over twenty-five had received a high school education in the mid-1970s. There was little opportunity for economic advancement, and many county residents stayed on the ranch for their entire lives. Traditionally the children of these individuals, boys especially, were encouraged to train in specific ranching techniques and take over their parents' roles. This system provided a constant labor supply for the ranches and helped to control wages to the benefit of the ranches' owners. This pattern began to change in the later twentieth century, but income and adult education levels in the county remained among the lowest in the state.

The county has a political machine and one school district:

Though the power of the machine has declined, the old elite still controls most local offices. In the early 1990s the county had one school district with two elementary schools. Two percent of the students were white, and 98 percent were Hispanic.

As to the Armstrong family, it acquired its ranch in 1882, right after the "skinning wars":

Pioneer Tobin Armstrong's ancestor, Texas Ranger John B. Armstrong, bought the beginnings of the Armstrong Ranch in 1882 with the $4,000 bounty he received for capturing outlaw John Wesley Hardin.

Kenedy County and the Armstrong ranch: where Dick Cheney, Karl Rove and other wealthy Republicans go hunting every year.