Complex sentence: They are usually tasked with keeping a fixed high value choke points clear of mines, for example the approaches to military ports and harbours, especially in a cold war context those used by a nation's ballistic missile submarines.
Split into simpler sentences: They are usually tasked with keeping a fixed high value choke points clear of mines, such as the approaches to military ports and harbours. In a Cold War context it was especially important to protect those ports used by a nation's ballistic missile submarines.