By Ashby Jones

Everyone born in a U.S. state or territory automatically gets U.S. citizenship—unless one happens to be born in American Samoa.

That exception is at the heart of a federal lawsuit filed against the U.S. government this week by five American Samoans and a Samoan organization based in California.

The Samoan islands, an archipelago in the South Pacific, have long been part of two countries.

The western islands belong to Samoa, an independent nation.

The easternmost islands make up American Samoa—granted to the U.S. under an 1899 treaty with Germany and Great Britain—and are home to more than 55,500 people, according to the 2010 U.S. Census.

Over the past century, Congress has passed a host of laws regarding citizenship and the five U.S. territories, which are subject to only parts of the U.S. Constitution and a patchwork of U.S. laws.

A 1917 statute, for instance, granted automatic American citizenship to people born in Puerto Rico, which became a territory in 1898. Similar laws later granted citizenship to people born in Guam and the Northern Mariana Islands in the Pacific Ocean, and the U.S. Virgin Islands in the Caribbean Sea. But people born in American Samoa are classified as “noncitizen nationals,” under a federal law first passed in 1940.

At that time, some in Congress were wary of granting full citizenship to those born in American Samoa, according to Rose Cuison Villazor, a law professor at the University of California, Davis, who is an expert on U.S. territorial law.

More recently, though, leaders in American Samoa have rebuffed efforts to extend American citizenship, partly out of fear it would lead the U.S. to challenge the territory’s unique communal land-ownership rules.

Currently, in order to become U.S. citizens and gain the accompanying benefits, such as voting and the right to hold certain government jobs, American Samoans have to apply for naturalization, a process that plaintiffs in a lawsuit filed Tuesday call “lengthy, costly and burdensome.”

In the suit, filed in federal court in Washington, D.C, plaintiffs claim that a law explicitly denying American Samoans citizenship upon birth violates the Citizenship Clause of the 14th Amendment to the Constitution. That clause states that “all persons born…in the United States, and subject to the jurisdiction thereof, are citizens of the United States.”

Several of the plaintiffs served in the U.S. military, according to the complaint.

“We fight America’s wars, we have men and women in the line of duty,” Leneuoti Tuaua, one of the plaintiffs, said in an interview. “Yet, the U.S. government doesn’t seem to care about that.”

Doug Kendall, [a] lawyer for the plaintiffs, said, “American Samoa is the only territory whose residents do not, under federal law, become U.S. citizens upon birth. American Samoans are given a sort of second-class status that the U.S. Constitution simply doesn’t allow.”

A spokesman for the U.S. Justice Department declined to comment. A spokeswoman for the State Department, also named as a defendant in the suit, declined to comment.

Peter Spiro, an expert in citizenship law and a law professor at Temple University, said the suit might run into problems. “There’s a long, historical tradition of U.S. territories lying outside the reach of the 14th Amendment,” he said.

That said, Mr. Spiro called the situation concerning American Samoa and citizenship “a real modern-day anomaly” that unfairly disadvantaged American Samoans. “It wouldn’t be hard for Congress to fix this, and in my opinion, it probably should.”