What was Trotsky's mistake?
.
He is the head of the Soviet delegation, and he withdraw from peace talk. From wiki:
the Central Powers... in the next fortnight seized most of Ukraine, Belarus, and the Baltic countries.
..On 23 February, the Central Powers sent new terms for peace. These terms included cession of
Dünaburg,
Livonia, and Estonia to Germany; cession of western
Armenia to Turkey; recognition of an independent Ukraine; immediate evacuation of Russian troops from Finland and Ukraine; and complete demobilization of the Russian Army
...Thus the new Soviet government agreed to terms
worse than those they had previously rejected.
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And with regard to Germany's collapse then that is hindsight. Germany could well have won the war if the spring 1918 offensive had been successful and IIRC that offensive actually almost succeeded.
They thought German will be collapse by revolution. Actually Trotsky and many others think the collapse of Germany will be much faster so they opposed the peace in the first place!
On the other hand, Germany also didn't get peace in the new conquered land, and still more than 1 million troops need to be held there when they should be on the western front.