When the battle of Dettingen took place, which occurred in 1743, where George II. commanded the British troops in person, Sir Andrew Agnew held the rank of lieutenant-colonel, and was appointed to the keeping of a pass at the outskirts of the British army, through which an attack of the French was apprehended. On this post of danger, accordingly, the knight of Lochnaw stationed himself with his regiment of Scots Fusileers as coolly as if he had been upon the boundary of one of his own farms in Wigtonshire. One day, while at dinner, he was informed that there were symptoms of a coming attack—that the enemy’s cavalry were mustering at no great distance. "The loons!" cried Sir Andrew indignantly; "surely they will never hae the impudence to attack the Scots Fusileers!" and forthwith ordered his men to finish their dinner quietly, assuring them that they would fight all the better for it. He continued eating and encouraging his officers to follow his example, until the enemy were so nigh, that a shot struck from his hand a bone which he was in the act of picking. "They are in earnest now!" he cried, and drew up his men to receive the enemy, who came on at full charge. They were a portion of the royal household troops, the picked and best -disciplined soldiers of France, mounted upon heavy and powerful horses, and armed with cuirasses that were buckled close to the saddle, so that the point of a bayonet could not easily find entrance within their steel panoply. Sir Andrew, who knew that it was useless to abide such an avalanche of man and horse, ordered his soldiers not to fire until they saw the whites of their enemy’s eyes, to take aim only at their horses, and open their ranks as soon as a charge was made upon them. This skilful manoeuvre succeeded as he had foreseen—the French horses were brought down in heaps, their riders easily bayonetted, and the far-famed household troops were driven back with heavy loss. After the battle, George II. observed, "Well, Sir Andrew, I hear that your regiment was broken; that you let the French cavalry in upon you." "Yes, please your Majesty," replied the gallant humourist, "but they didna gang back again."

The most important military service, however, in which Sir Andrew Agnew was engaged, was the defence of Blair Castle against the troops of the Pretender, during the insurrection of 1745-6. On the arrival of the Duke of Cumberland in Perth, to take the command of the royalist army, he found it necessary to occupy and garrison Blair Castle, the seat of the Duke of Athol, then absent, for the purpose of suppressing the disaffected of the district, and cutting off the communications of the rebels by the great roads between the southern and northern parts of the country. For this service Sir Andrew was selected, and despatched thither with a detachment of three hundred soldiers. Not only was no siege expected, but the place was ill fitted to sustain one; for it was scantily supplied with provisions, and had no artillery or military stores, while the soldiers had only nineteen rounds of ammunition per man. Of all this the rebels seem to have been apprised, and, accordingly, on the morning of the 17th of March (1746), Lord George Murray, lieutenant-general of the Pretender, Lord Nairne, Macpherson of Clunie, and other Jacobite leaders, resolved to recover the castle, and open their communications. They came, therefore, in great force, captured the detached parties that were without the castle, and suddenly appeared before the fort itself, while such a visit was neither expected nor desired. Most commanders in such a situation as that of Sir Andrew would have abandoned the fort as untenable; but he had not thus learned his military lessons under the great Marlborough: he resolved to defend it to the last, notwithstanding its impoverished condition, and thus give time for the collection of those forces by which the insurrection was soon after extinguished at Culloden. He therefore issued strict orders to his garrison, now reduced to 270 men, to save their ammunition with the utmost care; and, as there were no provisions in the castle but some bread and cheese, he commanded these to be dealt out in small daily rations.

As the obtaining of Blair Castle was of the utmost importance to the rebels, Lord George Murray, their ablest commander, commenced the siege in due form. He began by a summons to surrender; and knowing the old knight’s fiery temper, he wrote to him to this effect, not upon decent foolscap, but a piece of shabby grey paper. But who was to enter the lion’s den, and beard him with such a missive? No Highlander could be found to undertake the task, so that it was intrusted to a comely young servant maid of Blair Inn, who had found favour in the eyes of Sir Andrew’s young officers while they resorted there, and who naturally thought that they would not allow her to be harmed. She approached the garrison, taking care to avoid being shot, by waving the paper over her head like a flag of truce. When she delivered her credentials, she earnestly entreated the officers to surrender, assuring them that the Highlanders were a thousand strong, and would ding the castle about their ears; but this friendly warning they only received with peals of laughter, telling her that they would soon set these Highlanders a scampering, and visit her at the inn as before. No one, however, would deliver the summons to Sir Andrew, except a timid lieutenant of the company, whose nerves were further unstrung by the use of strong waters; but no sooner did the old knight hear the first sentence read, than he burst forth into such a storm of wrath, and uttered such fearful threats of shooting the next messenger through the head who dared to propose a surrender, that the lieutenant took to his heels, while Molly, who stood at the bottom of the stairs, and heard the whole, fled across the fields like a startled hare. She told her employers, waiting in the churchyard of Blair, the result of her mission, who laughed heartily at the rage of Sir Andrew. Still further to provoke him, and perhaps tempt him to a rash sally, they threw large stones at the walls, accompanied with biting jokes at his expense; but fiery though his temper was, and impatient of ridicule, he was too wary a soldier to afford them such an advantage. In the mean time, the more serious work of the siege went on with rigour, and, though the walls of Blair Castle were of great thickness, the assailants not only used common, but also hot shot, in the hope of setting the building on fire. The wood being luckily not very combustible, only smouldered as it received the balls. But the chief confidence of the rebels was to starve the garrison out, knowing how scantily it was supplied; and for this purpose they strictly blockaded the place, while their best marksmen were ordered to keep up a close fire wherever a man showed himself. This last incident suggested to the officers of the castle a practical joke at the expense of their worthy commander, whom they loved, feared, and laughed at when they dared. They therefore got one of his old uniforms; and having stuffed it with straw, and furnished the figure with a spy-glass, they placed it at a small turret window, where it looked like no other than Sir Andrew himself reconnoitring the enemy below. The rifles of the assailants were all brought to bear upon it, and the best marksmen of the Highlands continued to riddle this deceptive wisp, until Sir Andrew himself wondering why this point should have been selected for such a hot attack, ascended the turret, and there he saw this other identity standing under fire, as stiff, fearless, and imperturbable as himself! He was in a towering rage at the irreverent deception, and resolved that the perpetrator should not escape a share of his own joke. The wag was ordered to go to this spot so full of risk, and carry the puppet away, Sir Andrew gruffly pronouncing this retributive sentence: "Let the loon that set it up, just gang up himsel’ and tak’ it down again."

Beyond all military calculation, Sir Andrew Agnew, with miserably scanty means, had made good his position from the 17th of March to the end of the month. Longer than this, however, it was impossible to hold out, as the provisions of the garrison were exhausted, so that nothing seemed to be left them but a desperate sally, or immediate surrender. A faint chance indeed there might be of some messenger stealing through the leaguer, and carrying tidings of their condition to the Earl of Crawford, who was then at Dunkeld with a strong force of Hessians. This was now attempted, and the gardener of Blair Castle undertook to be the messenger. The gate was opened without noise; he stole out unperceived, mounted a horse, and rode cautiously down the avenue to the highway; but immediately a firing and pursuit commenced, and, on the following day, a Highlander was seen mounted on the gardener’s horse, so that the garrison thought he must have been either killed or taken. On the 1st of April, however, they were startled by an unexpected messenger; this was no other than Molly of the Inn, formerly the envoy of the rebels, who now came with the joyful intelligence that they had broken up their encampment, and gone away to Dalnacardoch. Sir Andrew, who was not only wary but short-sighted, would not trust the news, and abode a day longer in his hunger-bitten hold, when an officer arrived from the Earl of Crawford, to say that his lordship himself was on the road with his cavalry, and would arrive within an hour. Such was the case; for the gardener’s horse being alarmed at the firing, had thrown him, and been captured by the Highlanders, so that he had made his journey to Dunkeld on foot. When Crawford arrived, Sir Andrew drew up his soldiers to receive him, and thus addressed the Earl: "My lord, I am very glad to see you; but, by all that’s good, you have been very dilatory; we can give you nothing to eat." The Earl laughed good humouredly, and invited Sir Andrew and his officers to dine with him—an invitation that was never more welcome, perhaps, than at the present crisis. The summer-house in the garden was immediately turned into a dining-room, the table was plentifully covered with substantial dishes and excellent wines, and the half-starved and doomed defenders of Blair Castle were translated, as if by magic, into the regions of safety, hilarity, and good cheer.