As the title says, the film is set in 1917, somewhere between the beginning of February and the end of March. This coincides with Germany's Operation Alberich. The Somme Offensive, launched in July 1916, though incredibly costly, had resulted in a British victory. The fighting on the Somme devastated the core of the German army and pressure from the East and continued pressure from the British on the Somme front forced the new German commanders, Field Marshal Paul von Hindenburg and his Chief of General Staff, Erich von Ludendorff, to find methods to reduce the number of men required to hold the German line. The solution was to withdraw from more than 130 kilometres of existing trench lines to a new defensive line constructed 40 kilometres eastwards, known to the Allies as the Hindenburg Line. In some places more than 7 kilometres deep, the Hindenburg Line represented a doctrinal shift by the Germans in how they approached the defence of their lines.

The Somme had taught the German military that it could not match the firepower of the Anglo-French forces and that the rigid defence strategy that had stipulated the retention of the front line was not viable when faced with heavy concentrations of artillery. In response to this realisation and in an effort to conserve manpower, German defence underwent a doctrinal transformation. Rather than attempt to prevent the penetration of a rigid front line, defence shifted in favour of what is known as Defence in Depth. This doctrine advocates the ceding of ground to the attacker, disrupting the unit cohesion of the enemy, and relies on the inevitable loss of momentum before reserves and available firepower could be deployed to push the enemy back.

The Hindenburg Line was built in secret using forced labour from the occupied zones. Its front line was lightly held and was intended to collapse in the face of an attack. Attackers would have to cross through incredibly dense and deep belts of barbed wire, under machine gun fire. When they finally reached the main German line of resistance, they would be beyond their own artillery range and subject to German artillery. With the attackers weakened, the Germans could counterattack and retake the ground.

1917 is supposed to depict the secrecy of the German withdrawal and the construction of the new defence line. Schofield is supposed to warn the Devons that the line they are about to attack is the new defence line and that it was intended to draw them in to slaughter them.

As to the send part of your questions, I suspect that Cumberbatch's character was referring to the fact that even if they stopped the attack they were about to launch, the order to attack it would come eventually anyway. This is accurate. The British launched a new offensive, the Battle of Arras in April. This was directed against the Hindenburg line to the east of the french town of Arras. While mostly intended to draw German reinforcements away from the French offensive further south, it was still a major action and mostly failed to achieve it objectives, largely due to the strength of the new German positions. That said, it did see the Canadians successfully capture the tactically important feature known as Vimy Ridge, a truly remarkable feat considering what they were up against.