Submarine launched nukes are often considered the upper border of what would be a "tactical" nuke, so let's take one of the most common (W-76) in the US/UK arsenal. It has a fireball radius of 500 meters when detonating on the ground meaning the fireball covers an area of 0.79 km^2.
( Source: https://nuclearsecrecy.com/nukemap/ )
No, they aren't. There isn't a precise boundary for "tactical" nuclear weapons (which have gone out of favor recently), but a W76 definitely isn't one. Try something more like 10-20 kt. SLBMs are strategic weapons, plain and simple. And you're not just going to use fireballs to kill things, as that's inefficient.
On the other hand Marski, if you are tossing ground detonation nukes around in such numbers that the fireballs overlap you can be fairly confident that whatever formation existed there does so no longer.
Is this a planet you want to keep? Because if so, I'd recommend strongly against that. Groundbursts are very dirty.
Alright, since it seems I'm possibly the only one here who's actually gone through military service and is in reserve, I should explain couple of things for you folks.
I'm not military, but I've spent quite a bit of time studying nuclear weapons. Also, please stop being condescending.
Nuclear weapons aren't magical. They operate on laws of physics like everything else. You don't drop them somewhere and have everything go dead with 100% certainty. Outside the imminent blastzone and lethal air pressure area, you can survive by simply hiding in a foxhole. This means vehicles too; dig in, wait for the blastwave to pass, get out, prepare for fallout unless your mission requires you to keep moving.
This, at least, is true. Although it's not always the blast that kills you.
Modern militaries across the globe have plans and doctrines on how to operate in a nuclear war. They differ and vary from country to country but they all have one thing in common; spread out. Companies have minimum distance of atleast ten kilometers from each other to reduce potential losses from TNW (Tactical Nuclear Weapon) being dropped. Motorized, Mechanized and Armored military formations are the most vulnerable and easiest to find and are on top of the priority list just below HQ's for TNW strike.
No. 10 km between companies is way too much. Spreading out is good, but you can't fight like that, and most tactical weapons aren't that lethal. Seriously, show me one tactical weapon with any kind of lethal radius of even 5 km.
And armored/mechanized formations are the most survivable on a nuclear battlefield, not the least. AFVs are remarkably blast-resistant (hence enhanced-radiation weapons) and can carry NBC protection systems with them.
Infantry on the other hand are the least vulnerable (yeah, surprising), they can spread out as much as needed thus making it impossible to take out an entire battalion of infantry with just TNW's, infantry can take cover in absolutely anything or create some very fast. They aren't restricted by infrastructure and therefore can move anywhere and are difficult to detect from air or space.
And then are incapable of moving or, you know, actually doing anything useful. Leg infantry has its uses, but when it has no supporting infrastructure, it's not going to stand up to someone who does.