Which event allowed Germany to transfer soldiers to the western front?

Question

Soviet soldiers joined the war.
US soldiers joined the war.
Germany made peace with the Soviet Union.
Germany made peace with the United States.

Answer 1

The event that allowed Germany to transfer soldiers to the western front was that Germany made peace with the Soviet Union. This occurred with the signing of the Treaty of Brest-Litovsk in March 1918, which allowed Germany to reallocate troops from the Eastern Front to the Western Front.