We've seen phaser, torpedo, and even disruptor hits on planetary targets that don't really do all that much more than an equivalent sized shell hit from a battleship or cruiser. Unless the hit strikes something reactive (power generation system, or weapons storage) their aren't usually the huge secondary explosions either.
In DS9, single disruptor blasts at full power from at least several ships caused destruction to 30% of planetary crust from only a few seconds of bombardment.
Granted, they were paired with torpedoes, but each torpedo and disruptor blast caused a very large damage radius (easily equivalent or going way past 100 megatons)
Most other instances of planetary damage via phasers or photon torpedoes that did little damage were usually portrayed as coming from SF ships... which were highly modified - namely, they weren't set to full power and were modified to avoid atmospheric dispersal which could otherwise destroy life sustaining properties and any lifeforms on the surface.
There's controversy over the episode in question because the blast effects are flat (which is unintuitive and should be impossible in atmosphere), faster than light (again impossible, but not so much for Trek), and don't kick debris into space (which is illogical, but not out of the question for Trek).
However, the dialog is perfectly consistent in the episode, and not only do the Cardassians and Romulans think it is a realistic plan, but a Starfleet admiral at one point agrees that it should work as described. Then when it happens, a Romulan reports that it's working as stated. So, it's worse.
The weapons are not merely greater than 100 Megatons, like they might have been in TNG: "Skin of Evil" and VOY: "Rise" they exceed Gigaton scales of energy. DITL, unfortunately the only place I know of which has tried calculating the episode, has a couple estimates which range from 25,000 Megatons to 20,000,000,000 Megatons, or 25 Gigatons to 20 Petatons. For comparison, the Chicxulub crater (in real life) was created by a 240,000 Megaton (240 Gigaton) impact. One single hit by even the low figure figure would be enough to wreck a good portion of the planet for decades and kill tens or hundreds of millions of people.
This is why I find the Breen attack such a conundrum, and I'm glad to see you are aware of the episode. Actually, I was purposefully avoiding "The Die is Cast" because it takes the Breen attack from interestingly puzzling to utterly incomprehensible. There has to be shields at some scale which can dissipate planet vaporizing weapons, or there need to be damn near instant overwhelming defensive weapons, or both.
If a single hit got through it would be the end for Earth, so there has to be a shield either for the whole planet, or every continent, or at least for every population center. But it is preferable to be planet wide, otherwise the oceans might vaporize on the high end. Or, Starfleet has to be so absolutely sure of its defenses that the only danger ever is falling debris. But, the conquest of Betazed seams to counter that idea.
The problem is, even one shot from a well charged phaser should produce several kilotons equivalent of damage.
...If one wants to use a well charged phaser, that is. Some of the most impressive bombardments in recent history have involved serious pulling of the punches - the equivalent of a samurai cutting exactly two and a half locks of hair from the forehead of his opponent without drawing blood. Scary as hell.
What tactical goals could the Breen have aiming at? Remove SF HQ from existence? That would hardly carry practical tactical significance: Starfleet leadership would be well distributed during this widespread war, and its San Francisco elements well protected against anything lesser than a Xindi doomsday beam anyway. Maiming Golden Gate would actually have more impact on Earth's will and ability to carry on the fight!
The goal is show the Dominion can hit Earth, but to waste the opportunity to real damage is a pointless waste. Hitting Starfleet headquarters would have thrown the fleet into disarray, even if temporarily, as new personnel learn their positions. Assuming wide distribution of admirals is an assumption. We know there were admirals there as shown in other episodes.
Destroying the cities of the bay area would also deplete the Federation of minds, and cause greater public resistance to the war.
Putting a hole in a useless bridge, no matter how liked, won't do any of that.
Remember that collateral damage is easily avoided even when phasers of high power are used. The secondary effects from Nero's insanely powerful drill, for example, seemed to be limited to just a puff of vapor that did no harm to the SF Academy crowds ashore.
Collateral damage is easily avoided when the attacker wants to avoid collateral damage. The Breen had no reason to avoid collateral damage.
Nero's drill was a drill, so would have been designed to put as much energy down, and not out, as possible, in order minimize the amount of energy not drilling. We've seen numerous times that beams in Star Trek do not have secondary effects on contact with atmospheres.
I realized an error in the image.
Well, there's a curious error (?) in the DS9 image already: no shot-dots fall on the locations established in the actual footage of damage (Golden Gate and both the shores of the narrows)...
I meant with the map I made, not the canon map. The canon map actually has eight yellow triangles with black dots in the center of each, one of which is very near Starfleet headquarters. That one could explain minimal damage on Starfleet Academy if debrii from an impact there impacted on the Academy, but it doesn't explain why a better attack was not used.
This is why I think the Breen didn't actually touch any of San Francisco with weapons, why I think the city(s), or planet, either have shields, or only Breen ship debris caused the damage.
If there was no weapons fire against the city, and/or Breen wreckage was the only thing that might have pockmarked the landmarks, why is there damage at all? We have been shown that even the most benign civilian presence in space is capable of defending itself to some degree, against natural threats. And what would be more natural than inert objects falling from the skies? ENT already demonstrated Earth's ability to divert even largish comets!
My point is, if there were shields, then the damage could have been caused by bleed through, as in weapons energy with bleed through the shield and strikes the city anyway, causing reduced damage. It's similar to how weapons can hit shields and still cause system disruption.
Or, there were no shields and what hit the city was just ship debris. The quote of the incident says almost all the Breen ships were destroyed, implying there were too many for the assets there to handle completely. Meaning debris would have easily gotten through without being vaporized, since there wouldn't be time to waste on that action while enemy ships are still functional.
OTOH, we never quite hear of planetwide shields in Trek. Elba II
When discussed by the crew, they stated the shield at Elba II can be shot down, but it will destroy all life on the planet, even if they shoot the weak spot on the opposite side of the planet. The only way that would all be the case is if the shield were a normal phaser resistant barrier.
In "A Piece of the Action" we already know the phasers can be tune down enough to safely strike a city block with stun energy. So, striking an undefended generator without harming anything else would have been trivial.
I actually think the Breen raid represents a good balance in terms of what was established earlier on. We know that a single unopposed starship can destroy a planet ("A Taste of Armageddon"). We have seen a small fleet of ships achieve exactly that, in mere minutes, when the projection by the attackers was that it would take hours - apparently, the total lack of defenses made the crucial difference ("The Die is Cast"). We have seen defensive mechanisms that would explain the difference: orbital fortresses that make short work of mighty starships ("Tears of the Prophets"), shields and underground fortifications that make key assets (but not whole planets) impervious to attack by starships under certain minimum size and power ("Return to Grace"). And we know from DS9 and, through omission, from all the rest of Trek, that when major wars are fought, the combatants are unable to inflict strategic damage on each other's planets, and indeed never even bother to try, despite the ability to easily reach those planets under cloak or at high penetration speed.
Here the Breen exhibit nothing new in terms of tactical capabilities, just as Doolittle demonstrated no new capabilities in strategic bombing or naval-air warfare (indeed, he willingly revealed the fundamental weaknesses of the assets available to him!). The Breen show willingness to fail bravely - and that's what counts.
Timo Saloniemi
Now are not even in disagreement. My whole point is there has to be some sort of defense which prevented a proper strike on Earth.