这是个错句。因为句子不缺成分， what 就是多余，我怀疑 what 是 that 的误拼。这样，that 从句作 concluded 的宾语。

characterize 作“描述,  描绘” 解，常用于“characterize + 宾语 + as + n./adj. ” 结构。 意为“把......描述为......”，本句， the missile strikes 是宾语，as ...作宾补。

Iran had “concluded” that he characterised the missile strikes as “proportionate measures in self-defence”.

下面这个句子，用 what 才是对的。本句，what 是宾语，as ...作宾补。

Iran had taken and concluded what he characterised as a proportional response.