On Tuesday, the Supreme Court ruled 5-4, along ideological lines, that the family of a Mexican teenager who was killed across the southern border by a U.S. border agent cannot sue for damages.

Why it matters: The court’s decision avoids inviting more lawsuits from foreign nationals against U.S. law enforcement. The court noted in its opinion that “a cross-border shooting claim has foreign relations and national security implications.”

"Both the United States and Mexico have legitimate and important interests that may be affected by the way in which this matter is handled," Justice Samuel Alito wrote in the court opinion. "It is not our task to arbitrate between them."

Details: Border agent Jesus Mesa was on U.S. soil when he shot and killed 15-year-old Sergio Adrián Hernández Güereca in 2010, after the teen had run back across the border to Mexico.

“The gravity of this ruling could not be clearer given the Trump administration’s militarized rhetoric and policies targeting people at the border," ACLU attorney Lee Gelernt said in a statement. "Border agents should not have immunity to fatally shoot Mexican teenagers on the other side of the border fence. The Constitution does not stop at the border.”

What to watch: The ACLU has a separate but similar case on behalf of another Mexican teenager who was shot by a U.S. border patrol agent through the fence and killed in 2012.