Rumours that anti-fascist protesters planned to burn American flags on the Gettysburg Civil War battleground site led to armed far-right groups turning up in numbers on US Independence Day.
Posts on social media by supposed antifa leaders urged members to meet at the Pennsylvania site on 4 July.
In response, far-right groups assembled on the historic grounds on Saturday – but no adversaries showed up.
The holiday marks the US declaration of independence from Britain in 1776.
The posts that appeared on social media ahead of the Independence Day celebrations reportedly called for people to flock to the site in face paint.
Activists would “be giving away free small flags to children to safely throw into the fire”, the hoax call suggested.
“Let’s get together and burn flags in protest of thugs and animals in blue,” one anonymous Facebook post said.
There is no evidence to suggest that those responsible for the online posts are in any way linked to antifa – a loosely-affiliated network of mainly far-left activists.
Hours before the flag-burning was supposed to start, far-right groups gathered in a parking lot next to a Wal-Mart. Some were armed.
“It doesn’t matter if it’s a hoax or not,” Christopher Blakeman, who travelled to the Gettysburg site from West Virginia on Saturday, told the Washington Post newspaper.
“They made a threat, and if we don’t make our voices heard, it’ll make it seem like it’s OK,” he added.
In his Independence Day address on Saturday, President Donald Trump vowed to defeat what he called the “radical left” as protests against racism and police brutality continue.
Mr Trump said he would “fight… to preserve the American way of life”, while railing at “mobs” targeting historical monuments.
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In May, the president said the US would designate antifa, which he accused of starting riots at street protests over the death of African American George Floyd, a terrorist organisation.