A Texas law banning sanctuary cities would harm diplomatic relations between the U.S. and Mexico, Mexico argues in a brief submitted to the appeals court reviewing the law.

The brief, filed Thursday with the Fifth District Court of Appeals in New Orleans, says whenever state or local officials act improperly in upholding the sanctuary law, the U.S. federal government would be unable to resolve the problem or prevent similar ones. Thus it runs the risk "that actions of a state or its officials regarding immigration enforcement could irreparably damage U.S. foreign policy interests with respect to a particular country."

Texas' sanctuary law, known as SB 4, may be the toughest in the country. In addition to banning sanctuary cities — jurisdictions that choose not to comply with federal immigration enforcement — it allows local law enforcement officers to question the immigration status of people they detain or arrest.

And the law seeks to punish local government department heads and elected officials who don't cooperate with federal immigration "detainers" — federal requests to turn over immigrants possibly subject to deportation. Punishment could come in the form of jail time and penalties that exceed $25,000.

Passed by the majority-Republican legislature and signed into law by the state governor in April and May, SB 4 was promptly challenged in federal court, which ruled in August the measure could not take effect because it conflicted with U.S. immigration law.

In late September, an appeals court allowed partial implementation of the law, specifically the section requiring jail officials to honor all detainers, while the court considers the appeal.

Enter Mexico

FILE - Mexican border residents and members of the FILE - Mexican border residents and members of the Border Network for Human Rights (BNHR) hold up placards and a banner during a protest to reject border militarization and the deportation of children, outside a detention center in El Paso, Texas, Aug. 24 FILE - Mexican border residents and members of the Border Network for Human Rights (BNHR) hold up placards and a banner during a protest to reject border militarization and the deportation of children, outside a detention center in El Paso, Texas, Aug. 24

On record with the appeals court as opposing the law are most of Texas' largest cities, while 11 states and the federal government have signed on to support it.

Mexico joined the cities after the law raised concerns in the Mexican community.

The Migration Policy Institute estimates about 2.5 million people living in Texas were born in Mexico, and Mexico's brief says that SB 4 has caused them "fear, panic and uncertainty."

Between May and September this year, the number of phone calls to a Mexican government operation call center increased 818 percent over the same period in 2016.

Mexico's main argument, one likely to be echoed by other plaintiffs, is that U.S. immigration law is a federal matter, not a state one.

"Courts should carefully scrutinize any state immigration enforcement law that potentially requires foreign governments to negotiate or otherwise engage with the different states to protect the interests of its nationals," the brief said, adding that state laws have the power to "damage U.S. foreign policy interests."

A federal motion filed with the appeals court a week ago argued that state and local cooperation with detainer requests is a matter of public safety.

"Without such cooperation, such aliens would be released back into the communities, requiring federal officials to attempt more dangerous arrests on the streets," the motion reads.



The Fifth Circuit Appeals Court is considered one of the most conservative in the country. Oral arguments in the case are scheduled for November 7.