Question

In world war 1, for the allies what are the costliest year of the war?

Answer 1

The Hundred Days Offensive Jul–Nov 1918 was the costliest

The Ludendorff offensive other wise known as the spring offensive was a German offensive on the Western Front intended to win the war before the American troops that had begun arriving in France could fully deploy.

I'm unsure of how far they advanced

Kaiserschlacht was the last German offensive and was a series of German attacks along the Western Front.