The brigade saw service with the 46th Division on the Western Front in France and Belgium for the rest of the war, aside from a few weeks in Egypt, fighting at the Hohenzollern Redoubt in October 1915 after the failure of the Battle of Loos.

The attack was a failure and served only to gain the 46th Division a poor reputation until late September 1918 when, during the Hundred Days Offensive, it re-established its name during the Battle of St. Quentin Canal.

In the late 1930s, there was a growing need to increase the anti-aircraft defences of the United Kingdom and a reduced need for so many infantry battalions in the Territorial Army and so many of them were converted into other roles.

A new 138th Brigade was raised in mid-1939 when the Territorial Army was doubled in size throughout the spring and summer of 1939, due to the possibility of war with Nazi Germany becoming an increasing likelihood.

In April 1940, seven months after the outbreak of the war, the 138th Infantry Brigade, commanded at the time by Brigadier Edward John Grinling, DSO, MC, TD, a Territorial Army officer, and division, minus the artillery, engineers and other support units, were sent to France to join the British Expeditionary Force (BEF).

As a consequence, the division was battered in the Battle of France when fighting the German Army and, together with the rest of the BEF, was gradually forced to retreat to Dunkirk, where they were evacuated to England.

[8] After returning to the United Kingdom the brigade and division, due to severe casualties suffered in a futile attempt to stem the German advance, were both reformed with large numbers of conscripts and was sent to Scottish Command.

With the rest of the 46th Division, the brigade later fought in the Italian Campaign, suffering heavy casualties in the attritional fighting in Italy during the Salerno landings in September 1943 under X Corps, and later the fighting at the Monte la Difenso, Monte Cassino and the Gothic Line, where, during the Battle of Gemmano, the brigade suffered heavy casualties in what has been labelled the "Cassino of the Adriatic".