Having successfully pacified negotiations with England, in early December 1605 Henry was made colonel of the first Irish regiment in the Spanish army. To distinguish it from subsequently-formed Irish regiments, it eventually became known as the "Old Irish Regiment" () or the "Regiment of Tyrone". On 22 December, Henry wrote to
Robert Cecil, the English royal secretary, reconfirming his allegiance to
the English Crown. Since Henry had no military experience, his second-in-command, veteran soldier Edward FitzGerald, played an important role in the training of Henry and his regiment. FitzGerald was elected sergeant major due to his experience serving in the Spanish military for seventeen consecutive years; he had also trained Irish confederate soldiers during the Nine Years' War. He governed the regiment from a strictly military standpoint. Giolla Brighde O hEoghusa wrote on 19 September 1605 that Henry was then expected in
Flanders. Henry's regiment was sent to the Low Countries to fight against the Dutch Republic. Towards the end of 1605 he arrived in Brussels, where he served under commander
Ambrogio Spinola.. Other companies of Irish soldiers were formed in the Spanish military. After the
naval Battle of Dover on 13 June 1605, Captain Hugh Mostian's Irish company was absorbed into Henry's regiment. Mostian and Henry must have come into conflict as the former later resigned as captain. During Henry's 1606 campaign, his regiment saw action in Flanders and
Germany. Peace negotiations led to a
cessation of hostilities in early 1607. Hostility towards Tyrone increased sharply following Lord Deputy Blount's death in 1606. In 1607, Tyrone was due to travel to London to settle a legal case when information reached him that the government intended to imprison him, or possibly execute him, on account of his involvement in a treasonous plot. It is unclear from where he obtained this intelligence, whether this plot actually existed and if the government intended to arrest Tyrone. A group of confederate allies, including
clan chief Cuconnacht Maguire, organised an escape plan for Tyrone and his allies. Maguire travelled to Brussels where he contacted Henry and explained his plan to bring a ship to the
Donegal coast to facilitate an escape. With money from Albert VII, they were able to hire a ship at
Nantes and disguise it. The ship sailed from
Dunkirk to Tyrconnell in August 1607. On 14 September, many of Henry's family, including his father Tyrone and older brother Hugh, fled Ireland for the Continent in what is known as the
Flight of the Earls. Henry obtained a warrant for his family members so they could travel to Spain. At
Halle in late October, whilst his regiment was quartered in the
Bruges district, Henry met many of his kith and kin on their journey through Flanders. Henry remained in his father's company until they parted at
Namur. He remained in
Leuven for a time to watch over the noble children left behind, which included his younger half-brothers
Shane and
Brian and the Earl of Tyrconnell's son
Hugh Albert O'Donnell. English administrators viewed Henry's regiment with suspicion. They feared Tyrone would return to Ireland with this regiment to take his land back. Lord Deputy
Arthur Chichester discussed methods of destroying the regiment, which Edmondes called "the gunpowder regiment". Henry discovered English spies in his regiment. The spies were removed under a pretext, so that the English government would not realise their spies had been caught. News of the
Plantation of Ulster in 1609 prompted Tyrone to organise a formal reconciliation with James I. Tyrone asked Henry to travel to
Madrid to participate in peace negotiations. This timing was opportune for Henry, as the Spanish Netherlands and the Dutch Republic had recently signed a twelve-year truce. On 13 August 1609, Albert VII gave Henry permission to leave his post for six months to travel to Spain "on personal business and on business of his father". He was accompanied by three of his officers:
Owen Roe O'Neill (his first cousin, serving as a bodyguard), John Rath (captain of the Flight of the Earls) and Jenkins FitzSimons (who served Tyrone in Ireland). Albert VII described Henry in glowing terms: "[he has served] with so much valour and in such a manner that, both on this account and because of his very promising behaviour, he deserves all favour". Despite the combined efforts of Henry, Archbishop Conry and secretary Matthew Tully to organise Tyrone's reconciliation and return to Ireland, Philip III could not be persuaded. Henry's older brother Hugh died in
Rome in September 1609, which made Henry the eldest surviving son of Tyrone. Henry succeeded his brother as
de jure 5th Baron Dungannon. Around 1610, Henry and Fr.
Hugh MacCaughwell lobbied to have Hugh Albert O'Donnell's foster parents reinstated. == Death ==