Hugh O'Neil announced his resort to arms by a vigorous protest against the onslaught made on his friend O'Byrne. Without waiting for, or expecting any answer, he surprised the fort erected on the Blackwater which commanded the highway into his own territory. This fort, which was situated between Armagh and Dungannon, about five miles distant from either, served, before the fortification of Charlemont, as the main English stronghold in that part of Ulster. The river Blackwater on which it stood, from its source on the borders of Monaghan to its outlet in Lough Neagh, watered a fertile valley, which now became the principal theatre of war; for Hugh O'Neil, and afterwards for his celebrated nephew, it proved to be a theatre of victory. General Norris, on reaching Ireland, at once marched northward to recover the fort lately taken. O'Neil, having demolished the works, retreated before him; considering Dungannon also unfit to stand a regular siege, he dismantled the town, burnt his own castle to the ground, having first secured every portable article of value. Norris contented himself with reconnoitring the Earl's entrenched camp at some distance from Dungannon, and returned to Newry, where he established his head-quarters.
The campaign in another quarter was attended with even better success for the Confederates. Hugh Roe O'Donnell, no longer withheld by the more politic O'Neil, displayed in action all the fiery energy of his nature. Under his banner he united almost all the tribes of Ulster not enlisted with O'Neil; while six hundred Scots, led by MacLeod of Ara, obeyed his commands. He first descended on the plains of Annally-O'Farrell (the present county of Longford), driving the English settlers before him: he next visited the undertaker's tenants in Connaught, ejecting them from Boyle and Ballymoate, and pursuing them to the gates of Tuam. On his return, the important town and castle of Sligo, the property of O'Conor, then in England, submitted to him. Sir Richard Bingham endeavoured to recover it, but was beaten off with loss. O'Donnell, finding it cheaper to demolish than defend it, broke down the castle and returned in triumph across the Erne.
General Norris, having arranged his plan of campaign at Newry, attempted to victual Armagh, besieged by O'Neil, but was repulsed by that leader after a severe struggle. He, however, succeeded in throwing supplies into Monaghan, where a strong garrison was quartered, and to which O'Neil and O'Donnell proceeded to lay siege. While lying before Monaghan they received overtures of peace from the Lord Deputy, who continually disagreed with Sir John Norris as to the conduct of the war, and lost no opportunity of thwarting his plans. He did not now blush to address, as Earl of Tyrone, the man he had lately proclaimed a traitor at Dublin, by the title of the son of a blacksmith. The Irish leaders at the outset refused to meet the Commissioners—Chief Justice Gardiner and Sir Henry Wallop, Treasurer-at-War—in Dundalk, so the latter were compelled to wait on them in the camp before Monaghan. The terms demanded by O'Neil and O'Donnell, including entire freedom of religious worship, were reserved by the Commissioners for the consideration of the Council, with whose sanction, a few weeks afterwards, all the Ulster chiefs, except "the Queen's O'Reilly," were formally tried before a jury at Dublin, and condemned as traitors.