"The image of the simple wooden cross that originally marked the graves of American soldiers killed in World War I became a symbol of their sacrifice, and the design of the Bladensburg Cross must be understood in light of that background," Alito said.
"That the cross originated as a Christian symbol and retains that meaning in many contexts does not change the fact that the symbol took on an added secular meaning when used in World War I memorials."
Justices Sonia Sotomayor and Ruth Bader Ginsburg dissented from the majority opinion, with Ginsburg writing that "making a Latin cross a war memorial does not make the cross secular."
"Just as a Star of David is not suitable to honor Christians who died serving their country, so a cross is not suitable to honor those of other faiths who died defending their nation," Ginsburg wrote. "Soldiers of all faiths 'are united by their love of country, but they are not united by the cross.'"
The cross’s challengers included three area residents and the District of Columbia-based American Humanist Association, which includes atheists and agnostics. They argued that the cross, which is located in a suburb near the nation’s capital, should be moved to private property or modified into a nonreligious monument, such as a slab or obelisk.
"Our legislative efforts will be redoubled as the American Humanist Association works to strengthen the wall of separation between church and state, brick by brick," Roy Speckhardt, executive director of the American Humanist Association, said Thursday in a statement. "In the interim, our legal team will do what it can to narrow the breadth of this decision in courtrooms across the country."
Defenders included the American Legion, which raised money to build the monument honoring area residents who died in World War I. Other backers included the Trump administration and Maryland officials who took over maintenance of the cross nearly 60 years ago to preserve it and address traffic safety concerns.
"This decision simply affirms the historical understanding of the First Amendment that allows government to acknowledge the value and importance of religion," Michael Carvin, lead counsel for The American Legion said Thursday in a statement.
The Associated Press contributed to this report.