https://www.fas.org/sgp/crs/homesec/R41286.pdf
Although the military does not have primary responsibility to secure the borders, the Armed Forces generally provide support to law enforcement and immigration authorities along the southern border. Reported escalations in criminal activity and illegal immigration, however, have prompted some lawmakers to reevaluate the extent and type of military support that occurs in the border region.
On May 25, 2010, President Obama announced that up to 1,200 National Guard troops would be sent to the border to support the Border Patrol. Addressing domestic laws and activities with the military, however, might run afoul of the Posse Comitatus Act (PCA), which prohibits use of the Armed Forces to perform the tasks of civilian law enforcement unless explicitly authorized. There are alternative legal authorities for deploying the National Guard, and the precise scope of permitted activities and funds may vary with the authority exercised.
Were they actually sent? For how long?