1 A more imperial policy, dominated by expansionist militarist objectives, generated the tensions that ultimately led to the American Revolution.

过去分词短语是对主语的补充说明，为非限制性定语，相当于一个非限制性定语从句：which was dominated by...。

2. Freed by warming, waters once locked beneath ice are gnawing at coastal settlements around the Arctic Circle.

过去分词短语作原因状语，修饰谓语动词are gnawiing.

3. A push to develop the North, quickened by the melting of the Arctic seas, carries its own rewards and dangers for people in the region.

名词push在本句的意思为effort。过去分词短语对主语push作补充说明，为非限制性定语，相当于一个非限制性定语从句：which has been quickend by...。