As a show of force, moving nuclear submarines around doesn’t seem like a great play.

• A nuclear submarine’s strongest asset relative to a surface ship is that one can’t know where it is. It goes down, it doesn’t come back up again for half a year, that makes it hard to identify. Why give clues that narrow things down at all?

• Because it needs to stay hidden, you can’t show it to the party you’re doing the show of force to to prove that you’ve done the movement, which makes your words just functionally words — the only weight here is the credibility your words hold. (Which in Trump’s case may be one of, if not the, lowest credibility I’d personally assign to any historical US president.)

I mean, I think that moving literally any military asset other than submarines doesn’t have this issue. Surface vessels, aircraft, land forces, whatever.

The article does not make it clear what type of submarine — attack (SSN) or ballistic missile (SSBN) — is being referred to. A “nuclear submarine” refers to both, as the term refers to the submarine’s powerplant, not the armament. I am guessing, based on this response where the author says that he is not sure, that Trump never specified.

If the submarine in question is a ballistic missile submarine, it really doesn’t need to be anywhere particularly near a target to hit that target. US ballistic missile submarines fire Trident II SLBMs. WP has the Trident II range as “More than 7,500 mi (12,000 km)[8][9] (exact is classified)[10]”.

There are certain situations where you might want to fire an SLBM from less than that; you can fire it at a depressed trajectory to reduce the time until impact, which might be useful in a first-strike scenario where you want to destroy an opponent’s nuclear weapons before they can get off the ground.

https://scienceandglobalsecurity.org/archive/sgs03gronlund.pdf

SLBMs flown on depressed trajectories would have short flight times, comparable to escape times of bombers and launch times of ICBMs, thus raising the possibility of short time-of-flight (STOF) nuclear attacks. We assess the depressed trajectory (DT) capability of existing SLBMs by calculating the flight times, atmospheric loading on the booster, reentry heating on the reentry vehicle (RV), and degradation of accuracy for a DT SLBM. We find that current US and CIS SLBMs flown on depressed trajectories would have the capability to attack bomber bases at ranges of up to about 2,000 kilometers, and possibly at ranges up to 3,000 kilometers. To target bombers based furthest inland, a new high-velocity booster might be required, and attacking hardened targets would require a maneuvering RV (MaRY).

However, in that case, you probably aren’t going to want to hint that you are planning on doing so if you actually intend a first strike. Sure, you could try to so merely as a bargaining chip, but doing something that you probably wouldn’t do in an actual attack undermines the credibility of the threat and thus devalues the bargaining chip.

The US doesn’t really need to issue nuclear threats against Russia. It has strong conventional military superiority.

And there are some good reasons not to want to lower the bar for nuclear threats, as a convention. We don’t want to nudge the world closer to a situation where a nuclear war actually starts accidentally — not just in this scenario, but in later ones.