Here is an essay by S. Brian Willson about the Trident missile system:
The Trident missile is a submarine-launched ballistic missile (SLBM). Each nuclear-powered submarine is capable of carrying up to 24 Trident thermonuclear missiles that are independently targetable reentry vehicles (MIRV) with a range of 4000 nautical miles. Each of the Trident missiles is capable of carrying up to eight independently targeted nuclear warheads — meaning hydrogen bombs. Doing the math, eight times 24 is 192 warheads on each submarine. Each of these hydrogen bombs/warheads possesses 38 times more destructive power than the Hiroshima bomb. And each Trident submarine in total possesses the collective destructive power of hundreds of Hiroshima bombs indiscriminately murdering millions of civilians. Upon launching, total flight time is 25 minutes or less depending upon the target location. Trident’s accuracy and short flight time makes it a first-strike weapon.
Trident missiles are carried by 14 US Navy submarines, with US warheads (8 in Bangor, Washington; 5 in King’s Bay, Georgia, and 1 in Portsmouth, Virginia), and 4 Royal Navy Vanguard-class submarines, with British warheads, home ported at the British Navy base, Clyde in Faslene, West Scotland.
One Trident submarine can destroy an entire country. A fleet of Trident submarines is capable of destroying the world. These nuclear weapons threaten us as much as they do the other side. There’s nothing more suicidal than a nuclear weapon. Just one use of a Trident threatens to create a global nuclear winter, destroying the capacity for any human life at all to exist.
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I served on a US nuclear sub armed with Poseidon missiles. I never got the impression they were a first strike weapon. I don't think the officers and crews of Trident missile subs believe such a thing about their weapons, either.
On the other hand, the orders about when to launch (and at whom) come from outside the sub. Unrestricted communication with the outside world is impossible; being underwater limits the types of comms which can be received, and they are controlled by the captain of the ship. So in reality the officers and crew could become unwitting tools of a government which WANTS to carry out a first strike.
This is never supposed to happen, but I have little faith in the safeguards designed to prevent such a thing. If the President says "launch", they'll launch.
Do you trust your President? Has he ever lied to you before? Yes, I'm joking - of course he has ... about nearly everything.