[5] The CEP was shipped to France in early 1917, where the first groups received training in trench warfare and were equipped with British small arms.

On the night of 12 June, a more intense German attack was carried out against Ferme du Bois and Neuve-Chapelle sectors, being repelled by the 2nd, 3rd and 7th infantry battalions.

In the defence against this attack, 2nd Lieutenant Hernâni Cidade of the 35th Infantry Battalion stood out, being able to capture several German prisoners, including the captain who led the assault.

Another 2nd Lieutenant, David Neto from the 4th Infantry Battalion, only supported by his orderly, captured an entire German patrol made up of one officer and seven men on 13 December.

[11] These actions also made CEP one of the first units of the British First Army to comply with General Horne's request for the need to capture German prisoners for obtaining intelligence.

Another factor was that the UK diverted its whole sea transport capacity for use by American forces after April 1917 when the United States declared war to Germany.

[14] Another major problem was a gradual loss of manpower; by April 1918, 10% of the CEP's strength had become casualties, due to the constant attrition of front-line service, and almost half of the officers were no longer present at the front.

On 2 March, a strong German attack was made against Chapigny and Neuve-Chapelle sectors, with a heavy artillery, gas, mortars and machine-gun fire preparation.

On 9 March, under Captain Ribeiro de Carvalho, the 1st Company of the 21st Infantry Battalion, supported by a detachment of 25 sappers, launched a strong assault against the German lines in the Ferme du Bois sector area.

In this assault, the Portuguese forces were able to kill and capture a great number of German soldiers and to demolish several enemy fortified positions, suffering themselves 20 casualties.

On 18 March, a company made up of 100 volunteers from the 14th Infantry Battalion, under the command of Captain Vale d'Andrade, assaulted the German positions on the Neuve-Chapelle sector, capturing three prisoners.

Just three days before that happened, on the dawn of 3 April, a company of the 2nd Infantry Battalion, under Captain Américo Olavo, assaulted the positions of the German 81st Reserve Division in Chapigny sector, occupying its 1st and 2nd lines and demolishing them, before withdrawing under enemy artillery fire.

At about the 4am, the Germans started the Battle of the Lys with a violent bombardment, that lasted about two hours, made by 1700 artillery guns concentrated in front of the Portuguese sector.

The 4th Portuguese Brigade (defending the northern sector, with the 8th and 20th infantry battalions in the front line, the 29th in support and the 3rd in reserve) was attacked by the 42nd German Division.

By 8am, the left flank of the Portuguese forces started to be enveloped by the Germans, who penetrated the gaps opened by the collapse and fall back of the 40th British Division's 119th Brigade.

By 10:30am the Portuguese artillery batteries, which never stopped firing, even after the infantry positions defending them had been annihilated, started to be overrun by the German forces.

[23] The – overwhelmingly negative – historical verdict on the battle was epitomized by the comment that the Portuguese "ruined Ludendorff and saved their allies by running away";[24] whilst the German advance was superficially impressive, especially to begin with, it quickly lost momentum and stalled.

[25] However, it is likely that the collapse would have happened to any unit, not merely the weakened and demoralized Portuguese, as British troops in a similar position the previous month, in Operation Michael, had been overrun just as quickly.