While I'm not flaired I think I may be able to address at least some of your question. I'm going to presume that you are talking about the conflicts the Texas Rangers had with Mexicans in the early 20th century.

During the Mexican Revolution there was a great deal of unrest on both sides of the border. This arguably led to the Plan de Santiago rebellion. During this time period the Texas Rangers were expanded to enforce the law and maintain order in Texas.

I'll leave to others the discussion of the specific actions of the Rangers however, the charge of terrorism is one that I'm confident that I can actually provide a solid answer to in light of my assumption earlier. According to the Federal Bureau of Investigation Terrorism is defined in two ways.

International terrorism: Perpetrated by individuals and/or groups inspired by or associated with designated foreign terrorist organizations or nations (state-sponsored).

It doesn't appear that the Rangers meet this requirement so lets examine the second.

Domestic terrorism: Perpetrated by individuals and/or groups inspired by or associated with primarily U.S.-based movements that espouse extremist ideologies of a political, religious, social, racial, or environmental nature.

Now keep in mind that I'm using the current definition of Terrorism and I'm making the assumption that the FBI's definition is sufficient to determine the proper usage of the term. While in the second definition I could see an argument being made that they could be considered Terrorists I don't personally believe that their actions meet those standards however upsetting their actions were. They were actors of a state power acting to maintain law and order in a region that was if not officially but in the eyes of those on the ground in rebellion. Therefore, the terrorism charge is a bit of a reach in my opinion.

References

Revolution in Texas How a Forgotten Rebellion and Its Bloody Supression Turned Mexicans Into Americans Benjamin Heber Johnson Link

Terrorism FBI