Why are three of the UK's four nuclear deterrent submarines at sea?

The UK has deployed three Vanguard-class nuclear deterrent submarines simultaneously at sea for the first time in five years, according to The Times. The vessels reported as at sea are HMS Vanguard, HMS Vigilant and HMS Vengeance, while HMS Victorious is said to be in maintenance. At least one submarine is described as carrying ballistic missiles with nuclear warheads, and a navy source told The Times the deployment was “unusual.” The UK typically keeps at least one submarine on continuous patrol. The move has sparked questions about whether Britain has shifted its defense posture after Defense Secretary Wes Streeting warned last week that the country should be prepared for war within 1,000 days.