North Korea fired four ballistic missiles in the early hours of Monday morning, in what Japan's leader described as "an extremely dangerous action."

【翻译】北朝鲜在星期一凌晨发射了四枚弹道导弹，日本领导人形容为“一次极其危险的行动。”

【分析】in what Japan's leader described as "an extremely dangerous action"：总体看，这是一个介词短语作状语，说明发射的“方式”。

what Japan's leader described as “an extremely dangerous action”：是一个名词性从句，作in的宾语，what不需要翻译出来。具体来看，what作described的宾语，as “an extremely dangerous action”作what的宾补。

我举一个类似的例句：

I am not interested in what you have described as the most beautiful thing. 我对你所描述的最美的事物不感兴趣。