If the flimsy agreement Messrs Trump and Kim signed in Singapore is to turn out differently, as Mr. Trump insists it will，America must be clear-eyed and exacting in the detailed nuclear regime that it negotiates with the North.

结构分析：

If the flimsy agreement Messrs Trump and Kim signed in Singapore is to turn out differently, 条件状语从句。其中Messrs Trump and Kim signed in Singapore为省略that 的定语从句，修饰the flimsy agreement.

as Mr. Trump insists it will，比较分句 （传统语法认为是as引导的非限制性定语从句，as指代if从句的内容。

America must be clear-eyed and exacting in the detailed nuclear regime that it negotiates with the North. 主句。 其中that引导定语从句，that的先行词为the detailed nuclear regime, that 在从句中作宾语。