Not so. The nodes in the command chain have little if any ability to question the order they've been given. Officers are even selected based on their ability to carry out orders unthinkingly. In the US military, an order from POTUS is the ultimate command. The only part of the chain that matters is at the very top, specifically, the armed individuals who surround the President. They can choose to stage a coup or declare the President medically unfit.

They have little authority to stop the order, which is not the same thing as saying they have no ability. In practice they are human beings and a Stanislav Petrov situation is totally possible.

AFAIK the two man rule is to prevent a rogue soldier from launching, they have no authority to question orders from the president.

They are not allowed to override POTUS.

I also wasn't allowed to smoke pot, while enlisted.

Which is to say, the orders may not be followed by the persons at the switch. I make no claim that they will, or will not, be followed. I merely point out that rules aren't always followed.

Correction: Overriding POTUS is illegal and has consequences that do not include the planet being nuked.

They absolutely have the real authority (ability) to refuse the order, and then the real authority to prevent the nuke from being launched (by eating their key, shooting anybody else that comes into the room, shooting the key/console, etc).

Though as I understand it each missile site has its own control and 2 man system. Even if only 1 out of 10 obeyed their orders and launched it would likely be the end of mankind or at least civilization as we recognize it.