After a failed attempt with Robert Emmet in 1803 to renew the United Irish insurrection crushed five years before, he went into French exile and served with distinction in the army of Napoleon.

He joined the Society of United Irishmen in the city and became involved in a broader conspiracy to coordinate an Irish insurrection with a French landing in the British Isles and a rising by Jacobin radicals in England.

While its suggestion of a mass movement in England primed to welcome Napoleon, the "hero of Italy", was scarcely credible, it was proof of intent to invite and encourage a French invasion.

[1] Following the suppression of the United Irish insurrection in the summer of 1798, Allen joined Robert Emmet in seeking to re-establish the republican organisation on strictly military lines.

[4] After service in Spain, where he led a storming party in the capture of Astorga (1810) and was held captive by Spanish guerrillas (1811–12), he rose to the rank of chef-de-bataillon (lieutenant-colonel) (March 1814).