Complex sentence: The Polaris Sales Agreement was an agreement between the United States and the United Kingdom which formally arranged for the Polaris missile system to be provided to the UK to maintain its independent nuclear deterrent.
Split into simpler sentences: The Polaris Sales Agreement was an agreement between the United States and the United Kingdom which began the UK Polaris programme. The agreement formally arranged for the Polaris missile system to be provided to the UK to maintain its independent nuclear deterrent.