Copper was a vital strategic resource for the US in wartime - it was used as a casing for bullets – and while Montana was in no danger of attack from America's foreign enemies, this was a period of significant labour militancy, and there was significant chance that disputes might spill over into attempts at sabotage and strikes aimed at the disruption of supply lines. The I.W.W. (International Workers of the World, or "Wobblies") was especially active in mining communities, and one of its main organisers, Frank Little, was lynched in Montana in 1917, further inflaming tensions there.
In addition, Butte, which was where Bradley was stationed, was what Ossad describes as "a wild and rough frontier town... where the citizens packed guns." It was also home to a large number of Irish immigrants whose loyalty to the Allied cause had been placed in doubt by events in Dublin in 1916, when a nationalist rising took several days to put down and sowed the seeds that would shortly result in the Irish war of independence.
Indeed, in March 1918 the IWW planned a St Patrick's Day demonstration in Butte that, according to Axelrod, was "intended to explode into an outright riot aimed at shutting down the Anaconda copper mines." Bradley deployed the 86 men of the company he led in town with bayonets fixed to face off against "thousands of angry men, many armed with knives and brass knuckles."
The main instigators in Butte were Irish-American members of the Pearse-Connolly Club, tellingly named after two of the main leaders of the 1916 Dublin uprising. (Both men had been executed by firing squad in its aftermath, the injured Connelly infamously while seated strapped into a chair because he was unable to stand – quite possibly the single biggest propaganda coup the British ever gifted to Ireland's nationalists.) And, in September of the same year, Bradley's men raided the Metal Mine Workers' Hall in town, arresting "about 50 suspected members of the I.W.W. who were preparing fliers to call for a general strike."
Sources
Alan Axelrod, Bradley: A Biography (2008)
Steven L. Ossad, Omar Nelson Bradley: America's GI General (2017)