He volunteered for the 9th Battallion Royal Scots before the war being appointed as a second Lieutenant on 14 December 1909, while at the same time undergoing his law training in Edinburgh. He graduated in law in 1912 and was admitted as a Scottish Advocated later in the same year on Lieutenant on 17th December in the same year.
At the outbreak of war he was appointed staff-captain to the Lothian Brigade but rejoined his battalion in early 1915. In February 1915 Lieutenant Lyon found himself in the terraces near Glencose Wood, Ypres. After this first experience in the line he wrote two poems Easter at Ypres and Lines Written in a Fire Trench. A couple of weeks after this during the fiercest fighting of the second battle of Ypres he wrote two more On a Grave in a Trench and I Tracked a Dead Man Down a Trench.
On 23rd April 1915 he was mentioned in dispatches by Major John Ewing:
"C Company had come to a halt behind a hedge which was so thickly girt with barbed wire that men could not break through without great labour. Noticing this, Lieutenant Lyon very cooly stood up and, taking out his wire-cutters, began to make gaps. Machine-guns played with him, but withouy any sign of haste he proceeded with his task, never stopping until he had the rendered the hedge penetrable."
In May he was located in dugouts in Potijze Woods near the Menin Road about 200 yards from the firing line. On the 8th May the shelling was so fierce that trees were uprooted and tops sliced by shrapnel. Many men died in the onslaught and like Lyon many had not known grave.
A book of his poems Easter at Ypres 1915 and other poems was published in 1916 a mixture of poems he had written before the war plus the four mentioned here and others from the war period.
Of his brothers two of the others were killed in the war and a third died while a student at Haileybury.
I tracked a dead man down a trench
I tracked a dead man down a trench,
I knew not he was dead.
They told me he had gone that way,
And there his foot-marks led.
The trench was long and close and curved,
It seemed without an end;
And as I threaded each new bay
I thought to see my friend.
At last I saw his back. He crouched
As still as still could be,
And when I called his name aloud
He did not answer me.
The floor-way of the trench was wet
Where he was crouching dead;
The water of the pool was brown,
And round him it was red.
I stole up softly where he stayed
With head hung down all slack,
And on his shoulders laid my hands
And drew him gently back.
And then, as I had guessed, I saw
His head, and how the crown -
I saw then why he crouched so still,
And why his head hung down.
Walter Scott Stuart Lyon 1 October 1886 North Berwick, East Lothian, Scotland - 8 May 1915 Potijze Woods, Near Ypres, Belgium