Roberts, an Irish immigrant who became a wealthy businessman, rose quickly to a position of major influence amongst the Fenian Brotherhood before eventually taking leadership.

Under his direction, the Fenians in America moved to use Irish-American veterans of the American Civil War to invade Canada in the hopes that they could leverage this against the British Empire to negotiate for an Independent Irish Republic.

During the American Civil War, which raged from 1861 to 1864, thousands of Irishmen had joined the ranks of both armies, gaining military experience and were often now influenced by the idea of Radical Republicanism.

Roberts and his supporters argued instead that the Fenians should gather the Irish veterans and their resources, and instead attack Canada with the hopes of securing at least one Canadian region.

A subtext to Roberts and his faction's argument was that many veterans of the Union Army were angered by British support for the Confederacy during the war, and sought a measure of revenge by attacking Canada, regardless if this furthered Fenian objectives or not.

There Roberts' faction revised the Fenian Constitution by adding a preamble modelled on the Declaration of Independence as well as introducing of a number of checks and balances which effectively stripped O’Mahony of most of his power.

With the assembled Fenians enthralled by the report that the American government would not stand in their way should they raid Canada, Roberts and his faction would make a heave against O'Mahony.

Irishmen in every quarter of the land seeing that we are working instead of talking, the cause will go triumphantly forward[7] During the early summer of 1866, thousands of armed, uniformed and supplied Fenians crossed the border from the US into Canada as part of an invasion.

Although having some initial success against inexperienced Canadian militias, the Fenians were undone by poor logistics and the American government moving swiftly to block their efforts, fearing their activity would the pretext for a war between the US and the British.

Perhaps aware that this might be an accurate reading of sentiments, in September 1866 President Johnson ordered that the arms seized by the United States army from the Fenian during the invasion be returned.

His finances prevented him from seeking further re-election to that office, but after his time in the House he was a member of the board of aldermen of New York City in 1877 and was an unsuccessful candidate for sheriff in 1879. y.