They demanded (an) unconditional surrender.

他们要求无条件投降。

《牛津词典》

Even in case of an unconditional surrender, when the terms are dictated by the victor, they should nevertheless be embodied in a written capitulation as soon as practicable.

即使在 无条件 投降的情形，由胜方口述 投降条件时，亦应于最快时间内制作成书面文字。

youdao

Lee was sure Grant would not demand unconditional surrender.

VOA: special.2009.12.17

But he rejected General Shafter's demand of unconditional surrender.

VOA: special.2010.07.15

Grant demanded unconditional surrender.

VOA: special.2009.11.19

In 1945, the Allies rejected peace offers by German SS chief Heinrich Himmler, insisting on unconditional surrender.

CNN: Sunday,

The letter was given to General WHM Lowe after Pearse agreed to an unconditional surrender on 29 April, 1916.

BBC: Dublin may seek surrender letter

As U.S. strategists knew from having broken the Japanese military and diplomatic codes, there was virtually no inclination toward unconditional surrender.

FORBES: The Nuking Of Japan