There’s a few reasons. First off, you never know if an enemy might somehow stumble upon some sort of breakthrough that effectively neutralizes one of the legs of your triad. Much less likely that they come up with something that can neutralize all three.
Second, the different platforms do have somewhat different capabilities. Once a missile is launched, you can’t turn it around. Even if you could self-destruct it, if an adversary detects sub missiles and/or ICBM’s launched at it and thinks they might be nukes, there’s a pretty good chance they’re going to seriously consider launching their missiles in a retaliation attack before your missiles would have a chance of destroying theirs. Compare that to a bomber where you can have it in the air and making its way towards the target but still have much more time to call off the attack and/or change the target.
Third, land based ICBM’s are basically a ‘nuke sink’. Let’s imagine that Russia decided it wanted to launch a surprise all-out nuclear attack on the US. It’s likely that its initial targeting priorities for its nukes would be to destroy as much of the US’ nukes as possible, in order to minimize the counter attack against Russia. That means that Russia’s first strike is likely going to need to use up hundreds of their ICBMs and warheads attacking hardened missile silos that are generally in the middle of nowhere, instead of dropping them on more populated areas and/or other infrastructure. This makes it much harder to attempt to destroy the US’ ability to fight back, even if you could successfully pull off a surprise attack.
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