You don't need actual containers on actual container ships, so there's no need to pass the cargo through the usual channels. Any other kind of merchant ship will do - even a large hired fishing vessel owned by a dumb captain who thinks he's being paid not to ask questions.

Modern warheads are incredibly small, so the container is optional. I suspect it's perfectly possible to transport a warhead on a small yacht or cruiser. It's certainly possible on those floating palaces oligarchs like to flaunt.

Security on the Thames in London is practically non-existent. I don't know what it's like in Washington. But I do know the Pentagon has a nearby marina, so it's probably not that high.

Which means you can only really use this deployment mode if you make your first strike completely out of the blue.

I'm guessing that would be the aim - a completely unexpected decapitation strike of unknown origin.

The point is the old Cold War machinery seems completely defenceless against a sneak attack. TSA shoe-pantomimes impress me a lot less than some hint that someone has taken the possibility seriously and set up credible defences against it.