On 1 September 1756 he received a commission as lieutenant in the 37th foot, then serving in Germany, and in the following year returned to survey work in Scotland.

The siege was a difficult one, and Debbieg, who was a man after Wolfe's own heart, resolute and daring, giving little heed to rule or system where they interfered with his views of the best mode of attack, had many opportunities of displaying his valuable qualities.

He accompanied the army to Halifax, Nova Scotia, where he acted for a time as chief engineer during the absence of Colonel Bastide.

[3] In 1762, the French having seized Newfoundland, Debbieg accompanied the expedition sent to recapture it, landing with the troops at Torbay, nine miles from St. John's, under a heavy fire on 12 September.

He made plans of Barcelona, Carthagena, Cádiz, and Coruna, which are in the British Library, together with a manuscript entitled Remarks and Observations on several Seaports in Spain and France during a Journey in those Countries in 1767-1768.

On 29 August 1777 he was promoted to be brevet lieutenant colonel, and in the autumn was selected as chief engineer on the staff of Jeffrey, Lord Amherst, commander-in-chief.

He constructed a military bridge across the Thames between Tilbury and Gravesend, formed of barges so arranged that a cut could be easily made for navigation.

He invented a movable chevaux de frise and a machine on wheels for defending a breach, an engraving of which is given in Grose's Military Antiquities.

Little time was available; but when, five days later, the riots commenced he had been able to take effectual measures for the protection of the Bank of England, the British Museum, and other public buildings and offices, as well as the New River Head.

The riots ceased on 7 June as soon as the king ordered active military measures, but Debbieg continued to exercise his metropolitan responsibility until early in July, when trade and tranquillity were completely re-established.

[5] At the manœuvres of 1780 the king complimented Debbieg on the rapidity with which he threw three bridges across the Thames below Gravesend, by which the whole army was quickly transferred from Essex to Kent.

In October Debbieg submitted to Lord Sandwich a proposal to close Gillingham Creek, and to improve the navigation of the Medway at Chatham.

[5] On the Duke of Richmond becoming master-general of the ordnance in March 1782, Debbieg, who had had some passages of arms with him on the subject of defence, and had been attacked by him in the House of Lords in the previous November, found, or fancied he found, his position slighted and his official representations ignored; and when the duke obtained a royal warrant for the reduction and reorganisation of the Royal Engineers in 1784, by which the emoluments of the colonels were very largely reduced, Debbieg's hot temper and outspokenness got the better of his judgment, and he wrote a private letter to the duke, couched in such strong terms that he was tried by a general court-martial, and sentenced to be reprimanded.

Having worked out and submitted a scheme of considerable merit and breadth of view for the defence of the kingdom, of which no notice whatever was taken, he wrote another intemperate letter to the duke, dated 16 March 1789, and published it in the Gazetteer.

This incident is referred to in the Rolliad in the lines beginning: Learn, thoughtless Debbieg, now no more a youth,The woes unnumbered that encompass truth.