In early 1918, American troops were reaching France and beginning to make an impact on the ebb and flow of the war. While the previous combatants had been largely deadlocked for years, fresh American troops could turn the tide of otherwise evenly matched fights.
Germany was on the losing side of this power shift and needed to win the war before more American troops and equipment could arrive. A grand offensive was planned that would come to be known as the Fourth Battle of Ypres or the Battle of Lys.
If successful, it would have forced the British back to the channel ports and possibly caused an evacuation like that in nearby Dunkirk 22 years later.
A two-day artillery bombardment preceded an attack on April 9, 1918, that drove the Portuguese defenders in the Ypres Salient back five miles and cost 7,000 Portuguese lives.
British troops in the area were forced to pull back and cover the gaps of the withdrawing Portuguese soldiers and nightfall on April 9 found them in a precarious position. They held the high ground that the Germans desperately needed and they were outnumbered. The British 19th Division was attempting to hold off a concerted attack by the entire German Fourth Army.
In this brickfilm, a stop-animation movie made almost entirely with Legos, YouTube user Snooperking recreates that disastrous morning for the allies in April 1918 as the British attempt to hold the line and prevent the Germans taking the high ground.
Luckily, while the middle weeks of April 1918 were disastrous for the British in terms of lost territory, they did bleed the Germans heavily for every yard of territory lost. The German offensive stalled and was called off at the end of April. German losses during the attack allowed for their stunning defeats a few months later as Allied forces, bolstered by American reinforcements, went on the offensive.
(h/t Doctrine Man)