Operation Lone Star (OLS) is a joint operation between the Texas Department of Public Safety and the Texas Military Department along the Mexico–United States border in southern Texas. The operation started in 2021 and is ongoing. According to Texas governor Greg Abbott, the operation is intended to counter a rise in illegal immigration, the illegal drug trade, and human smuggling. Between fiscal year 2020 and fiscal year 2021, migrant apprehensions had risen 278% along the US–Mexico border. According to the governor's office, OLS has resulted in 513,700 migrant apprehensions, 44,000 criminal arrests (including 38,600 felony charges), and 489 million doses of fentanyl seized. As of April 2022, OLS was spending approximately $2.5 million per week and was expected to cost approximately $2 billion per year. Approximately 10,000 National Guard members were deployed in support of OLS at the height of the operation, with around 6,000 deployed as of November 2022. One year after the start of Operation Lone Star, Texas saw a 9% increase in migrant encounters along its border with Mexico, compared to a 62% increase in Arizona, California, and New Mexico along their respective borders with Mexico. As of June 2024, the Department of Public Safety has estimated a 74% drop in illegal border crossings since the start of OLS.

OLS has drawn support from many Republican Party state governors. OLS drew criticism from the federal government under then-president Biden, Democratic Party governors and mayors, and migrant advocates for its treatment of migrants, including the withholding of water and orders to push migrants back into the Rio Grande. Migrants have had a more difficult time crossing areas of the Rio Grande due to razor wire set up by OLS, leading to some migrants becoming injured and/or captured in the wire. Human Rights Watch (HRW) has criticized high speed pursuits in counties implementing OLS, which it attributed to causing 74 deaths. Texas officials and national guard members have also voiced concerns about hardships sustained during deployment in support of OLS.

According to the governor, 119,200 migrants were voluntarily bused to sanctuary cities across the United States as of June 2024. A few migrants were also flown directly to these cities. This has resulted in migrant crises in cities like New York City, Chicago, and Washington, D.C., as local resources are stretched thin to handle the new arrivals. Local officials in the sanctuary cities have criticized the busing program and responded by requesting federal assistance, fining charter bus companies carrying migrants, and sending migrants to other cities.

In January 2024, Texas officials seized control of Shelby Park in Eagle Pass, which was frequently used by United States Border Patrol to process new migrant arrivals. Border patrol agents were generally prohibited from the park, except to access a boat ramp in the park after three migrants drowned nearby while crossing the Rio Grande. This led to a standoff between federal and state officials. The Biden administration has said that it would refer the dispute to the United States Department of Justice if access was not restored for border patrol agents.

Starting with "Operation Linebacker" by former governor Rick Perry, the State of Texas has been launching border security operations with increasing escalation since 2005. These operations were limited in scope due to the exclusive authority of federal immigration agents to deport migrants. Operation Lone Star was launched in 2021 to respond to the surge in border crossings, which Governor Abbott attributed to the Biden Administration's policies on immigration. In fiscal year 2021, enforcement actions by U.S. Customs and Border Protection, including detentions and arrests of migrants, rose to over 1.9 million, a 202% increase from fiscal year 2020. Meanwhile, a 278% increase in migrant encounters was seen at the southwest border from fiscal year 2020 to 2021, which continued rising into 2022.

Operation Lone Star differed from previous border operations due to the authority granted to state law enforcement officials to arrest migrants in border counties for offenses such as criminal trespassing and human smuggling. OLS efforts to empower local law enforcement to act against undocumented migrants have been complicated by the traditional delegation of immigration enforcement powers to federal officials. In Arizona v. United States, the Supreme Court overturned an Arizona law penalizing illegal immigration at a state level.

As of June 2023, polling from the Texas Politics Project at the University of Texas at Austin indicated that 59% of Texans backed the increased law enforcement deployments and border spending.

The mission faced public criticism, including from state officials, following reports of pay delays, poor working and living conditions, a lack of proper equipment and facilities, and multiple suicides and suicide attempts among service members. According to reporting in the Army Times, soldiers were being housed in what it describes as cramped quarters, in converted recreational vehicles and semi-truck trailers, and also faced shortages in cold weather uniforms, medical equipment, and portable toilets. According to the Houston Chronicle, this was further compounded when it coincided with state cuts in educational benefits for service members to address budget shortfalls, reducing available tuition assistance by more than half. Some Texas Air National Guard members deployed in support of OLS have also criticized the operation's planning and execution, with nearly 30% of 250 participants in a 2022 Air National Guard survey reporting frustration with the operation's length, haste, and involuntary nature.