Provided, of course, that he is willing to encase the attacking country in nuclear weapons until there is nothing left of it. When both sides have nuclear weapons they might as well not have them, because there is no point in using them because it will only bring mutual destruction and that is not what war is about.
That's why I said if the Americans are stupid enough to use it. Because that way the U.S. automatically erases itself from existence. Such a conflict will be exhausting but losing does not equal annihilation like in MAD. You can always recover from defeat in a conventional war. From a nuclear one not necessarily.
Only insane , fanatical or desperate people are capable of using weapons in MAD, preferring to be dead than under someone else's boot. An ordinary person is unlikely to part with his life and be remembered as the gravedigger of the world.
You are demonstrating a severe lack of understanding about how nuclear weapons, doctrine, and policy, work.
First off, there is a difference between tactical and strategic nuclear weapons. Tactical weapons can be deployed by something as small as an MLRS, and be an alternate payload for a strike fighter, missile cell on a ship, or even used as a glorified remote-detonation mine.
Strategic weapons are the big bombs and missiles that can generally be detected from long ranges, and are what provoke immediate responses when used.
Second off, there are only two nations in the world with currently-relevant systems regarding intercepting strategic nuclear missiles, the USA and Israel. If there actually
is a strategic nuclear exchange between the USA and other major powers, the USA is the only nation with a
chance of coming out partially intact. The exact odds are unknown, but there is at least a
chance, and with the current development arc of technology, the US's capabilities in that regard are going to be improving steadily, while it'll be decades or never before anyone else has the capability.
On top of that, given the US has a habit of developing technology to counter what it itself is already doing, odds are decent that by the time anyone else comes up with said capabilities, American penetration aids and the like will already be designed to overcome them. Not guaranteed, but a solid chance.
Third off,
responding to tactical nuclear weapons with all-out strategic launch is standing policy for nobody.
If a hostile power tries to send their military across the Atlantic or Pacific, and the US responds with tactical nuclear weapons that blow the shit out of said invasion fleets, this is not an automatic 'and now everyone launches nukes and everyone dies.' It's also important to note that due to shorter travel distances and different delivery systems, tactical nukes will generally have detonated before there's even the
chance at a strategic response, meaning decision-makers will have to make said decisions after the fact, not when tactical weapons are in the process of closing on their fleet.
Further, the very mutually-assured destruction you are talking about actually makes it
less likely that tactical use will result in a strategic response. If the US tac-nukes the fleets, the enemy nations are out their expeditionary forces, but the nations themselves are still
fully intact. If they then reply with strategic weapons, the US does likewise, and
then everybody in major cities dies.
This means that MAD actually works
against the invaders, because they can either accept the loss of a big chunk of their military, or they can all die in exchange for killing tens of millions of Americans out of sheer spite. They gain
nothing except spiting America by launching strategic weapons.
The same is largely true for any other nuclear power. It is generally understood that attempting a conventional invasion of a nuclear power invites
defensive nuclear response, which is why it'd be suidicing your military for little to no gain. India and Pakistan are something of an exception to this with each other.