He is listed in the 1914 9th Bn Yearbook as Colour Sgt of "E" Coy. (8-company system)

He was awarded the Territorial Force Efficiency Medal in January, 1914.

After being mobilised since August, 1914, Percy Byrne and the 1/9th King's embarked for France on 12th March, 1915. There they joined the 2nd Brigade, 1st Division and were no doubt looked upon with some scepticism by the regular soldiers that they were attached to in the early days in the trenches.

They suffered heavily in their first battle, Aubers Ridge, in May, 1915, even though they were only used in a support role and their attack was called off before they went over the top. The survivors now saw themselves as experienced, which was just as well, for even harder times were to come.

In September, 1915, the battalion prepared for the forthcoming major offensive, the largest of the war so far - the Battle of Loos.

From "The 9th King's in France" by Capt. E.H.G. Roberts, M.C. & Bar:
"The Battalion and the London Scottish formed a body called "Green's Force," to which was given as a first objective the German front line trenches in the vicinity of Lone Tree, as this objective was left uncovered by the diverging advance of the 1st Brigade on the right and the 2nd Brigade on the left. In the grey light of the morning on the 25th September the British guns opened with a furious fire after many days of artillery preparation. The great battle had begun. For some time, and according to orders, the Battalion remained in its position. It was not to advance before 8 0 a.m. At this time the men left the assembly trench to move over the open to the front line. The enemy machine gunners had the range, and several were wounded almost on leaving the trench. The advance was made by sectional rushes, each section seeking what cover there was. Those who were wounded while actually advancing in many cases received slight wounds, but those that were hit while lying down were generally killed, as the bullets struck them in the head or traversed the vital organs for the length of the body. It required a courageous heart to advance seeing one's comrades thus desperately wounded or lying dead. The shell fire was not heavy, and few casualties were attributable to it. Lieutenant Colonel Ramsay led the attack in person, and he was easily recognisable by the wand which he carried. One of the Battalion machine guns was pushed forward about 2 0 p.m. and under the covering fire it afforded the advance was continued. The advance had been slow and losses were severe, but at 3 30 p.m. the men had succeeded in establishing themselves in one line about a hundred yards from the German trenches. A few minutes afterwards the Germans surrendered, and between three and four hundred prisoners were taken. They chiefly belonged to the 59th and 157th Infantry Regiments. A harvest of souvenirs was reaped by the men, many of whom secured the then coveted Pickelhaube helmet. The prisoners were sent to the rear, and the Battalion continued the advance and ultimately established a line on the Lens Hulluch Road. It is to be observed that the Battalion was the only one that got its field kitchens up to the village of Loos on the first day of the battle. At 4 0 a.m. next morning the Battalion was withdrawn to the old British line. Later in the day it moved forward to the old German trench system as reserve in the continued operations, sustaining several gas and shell casualties. On the 28th September the Battalion moved back to Mazingarbe, as the men thought, for a rest. They were soon disappointed. At 7 p.m. on the same day orders were received to take up a position at the Slag Heap or Fosse at Loos, known as London Bridge. At 9 0 p.m. the Battalion left its billets in a deluge of rain and marched back to the line in splendid spirits in spite of the fatigue resulting from the recent fighting. It was relieved from the trenches on the 30th September, and after one night spent in the ruined houses of Loos went to Noeux les Mines for a few days to re organise and re equip. On the 7th October the Battalion, now reduced to a rifle strength of only 425 men, returned to the front line which was alongside the Lens Hulluch Road to the north of Loos. The trench had evidently once been the ditch on the side of the road. It was very shallow, and it was decided to deepen it the next night as the men were too tired after their long march. This was a good resolution, but it was not carried out. The enemy commenced next morning about half past ten with heavy shell fire. In the afternoon it became intense and an attack seemed imminent. There was no shelter in the shallow trench, as there had not been sufficient time to make any dug outs. The men could do nothing but wait. Minutes seemed hours. The shelling appeared endless. So terrific was the enemy fire that it was doubted by the artillery observers in rear whether any of the front line garrison was left alive. All who might be lucky enough to escape physical destruction would at any rate be morally broken. The Germans who had concentrated in the Bois Hugo attacked about 4 30 p.m. They were repulsed by rifle and machine gun fire, and it is gratifying to know that two of the Battalion machine guns caught the enemy in enfilade and executed great havoc. So exhausted were the men that the Battalion was relieved that night and taken to the neighbourhood of Le Rutoire Farm." Company Sergeant Major Byrne was in the front line, with his men, throughout the battle until he was hit in the right arm by a machine-gun bullet at some time on the 8th October. He was evacuated from the battlefield, through the system of medical facilites, to a base hospital on the French coast before finally being evacuated to a hospital in the UK. Such was the seriousness of his injury that his right arm was amputated - his war was over.

The 1/9th King's battalion war diary for 7th November, 1915 states "Company Serjeant Major P.P. Byrne awarded Medaille Militaire". The formal announcement of this award was made in the London Gazette of 24th February 1916.

It is often stated that foreign awards were often issued to those in the British forces who had been awarded gallantry medals by their own commanders but it appears not to be the case in this instance as Percy Byrne was not recommended for the DCM until March 1916, almost six months after the announcement of his Medaille Militaire.

Citation Date: 1st March, 1916
No. 115 CSM Percy BYRNE
1/9th Liverpool Regt
"For Conspicuous gallantry and devotion to duty during the fighting from 25th to 30th September, 1915. During the attack near LE RUTOIRE on 25th September, when his Company Commander and most of the Officers of his Company were killed or wounded, he took command of a majority of the Company and pushed forward to the attack. On the 30th September, when in a position on Hill 70, he brought in, under fire a wounded man of another regiment who had been lying out for three days in a dangerous position."

The award of the DCM was published in the London Gazette on 3rd June 1916 and the citation, abbreviated, as was usually the case, on 21st June 1916.

The London Gazette version of the citation:

An article published in the "Territorial Gazette" (date not yet established) reads:
"1/9th BATT. LIVERPOOL REGT. An interesting ceremony took place last week at Blackpool, when Col. Cratton, commanding the third line groups, presented three men of the 1/9th King's (Liverpool Regiment) with decorations won during the assault on Loos on September 25. The men in question were Co.-S.-M. Byrne, on whom the president of the French Republic had conferred the Medaille Militaire, and Sergt. P. Williams and Pte J. Smith, who had been awarded the D.C.M. In making the presentations Col. Gratton said that as a King's man he felt a special pride in being able to associate himself with the present ceremony, for he recognised how well the 1/9th King's had done. In congratulating them he was only echoing the praises of Sir Douglas Haig, who had published in a special order of the day his appreciation for the deeds of the 1/9th King's, they being, he considered, equal to the 1st Battalion, who had left Aldershot at the beginning of the war under his command. The Medaille Militaire is one of the highest honours conferred by France, and is practically synonymous to our V.C. It is held by very few men in the British Army, and Liverpool may feel justly proud of Coy.S.-M. Byrne, who has seen 16 years' service with the 9th King's, he being the only man in the West Lancashire Division who has received the coveted honour."

He was discharged from the Army on 21st October 1916. The reason given on the Silver War Badge Roll is 'Sickness', but without an arm this should obviously be 'wounds'. He was issued with Silver War Badge number 60235.

His service papers appear not to have survived.